Chapter 3: seed propagation.
cotyledon damping off direct-seeding emergence etiolation [F.sub.1] hybrid geotropism harden-off hybrid imbibe imbibition mycorrhizae open-pollination pasteurization prothallus radicle scarification sorus (sori) stratification viability
In the previous chapter, the origin and development of seeds were explored and examined. Reproduction by seed is the way the genetic material of plants is recombined and passed on to the next generation. But a developed seed is only the beginning. Seeds have specific requirements for germination and growth that include optimal temperature ranges, moisture and humidity levels, light, and a suitable root medium. An understanding of these requirements and providing all of them at optimal levels contribute to the green thumb of the horticulturist. In addition to these basic environmental conditions, one must also realize that plants experience several stages of growth and that their requirements often differ at each stage. Perhaps the most sensitive stage is during and immediately after germination, when the new plant is rapidly using the stored food in the seed and beginning to grow roots and leaves to extract nutrients and make its own food (Fig. 3-1).
WHY GROW FROM SEED?
Starting your own seeds is preferred over buying plants from a greenhouse or nursery for several reasons. Perhaps the main reason is the selection. You can find heirloom seeds and specialty seeds for many plants that you will not find at the local nursery or greenhouse. You may wish to grow native wildflowers or rare tropical plants, you may desire to grow your own luffa sponges and birdhouse gourds, or perhaps you want to grow tomatoes like the ones your great-grandmother had in her garden. All of these plants and many more various species and cultivars are available from a plenitude of seed sources.
Another reason to start your own seeds is that you will have plants available when you want them. Although many nurseries and greenhouses provide cool-season crops in the spring, they are harder to find in the fall. Yet, many vegetable crops will produce quite well in the fall and even in the winter if you live in an area with a mild winter climate.
WHERE TO GET SEEDS
Seeds may be purchased from catalogs, from on-line sources, through seed exchanges, or at local nurseries and greenhouses (Fig. 3-2). The quality of seeds varies, so it is best to get them from reputable sources. You may also collect your own seeds or get them from your friends' gardens.
Seed performance is important. Seeds are living organisms that can survive fairly well under optimal conditions, but over time seed survival decreases. Under less-than-optimal conditions, seeds lose viability more rapidly. A quick and easy method to judge seed performance is germination percentage. A hundred seeds can be germinated in a moist paper towel that is placed inside a plastic bag (optimal temperatures should be provided). Within a specified time (usually 1 to 2 weeks for most garden plants), the total number of seeds germinated should be counted. (These can also be counted and removed daily while they are germinating.) This germination percentage will give a good indication of the overall seed quality.
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There is an international entity, the International Seed Testing Association, that seeks "to develop, adopt and publish standard procedures for sampling and testing seeds." The organization also wants to "promote uniform application of these procedures for evaluation of seeds moving in international trade."
Reputable seed companies test their seeds and can provide information to the purchaser about the quality of a particular seed lot (see box). In some cases, testing is required by law, and seed lots must be labeled with pertinent information that may include the germination percentage, the amount of noncrop seed in a package (purity), and whether there are any weed seeds. For the homeowner who purchases garden seeds from a garden center or nursery, the seed package provides a different sort of information. It may list details about how to plant the seeds: how deep, how far apart, and how far apart the rows should be. It may also tell how much time is required for a mature crop.
COLLECTING YOUR OWN SEEDS
One of the fun aspects of horticulture is collecting seeds from the wild or from a garden and then growing plants from them. If you do collect your own seeds, there are several factors to be aware of. One is that seeds contained in a dry, dehiscent fruit may be lost on the ground when the fruit splits upon ripening if you do not collect them early enough. You may be able to harvest the seeds before the fruit splits. Or you may be able to place a bag over the ripening fruit and fasten it with a string or rubber band, and then collect the seed once the fruit has fully ripened. You should also be aware of the pregermination requirements of many species. Many woody plants and herbaceous perennials, especially temperate-region plants, require a period of cold temperatures for complete seed development (Table 3-1). Seeds are collected and placed in a cool moist environment for a few weeks or longer, depending on the species. This process is known as stratification.
Some seed coats are very hard and impenetrable by water. Seeds cannot take up, or imbibe, water unless the seed coat is broken. This occurs in nature with the process of freezing and thawing or rubbing against gritty soil particles. Sometimes seed coats are broken down when they pass through the digestive system of birds or other animals. If you are dealing with a seed with a hard coat, try rubbing it on sandpaper or a metal file. Chemicals are sometimes used, although they can be quite dangerous to handle. For example, seeds may be treated in sulfuric acid to break down the hard seed coat. Hot water can be used in some cases. With hot water scarification, seeds are placed in 170 to 210[degrees]F water. They are allowed to continue soaking for 12 to 24 hours after the water cools. Whether by hot water or mechanical or chemical means, the process of breaking down hard seed coats is known as scarification.
Seed companies may pretreat seeds to provide for optimal germination rates and reduced times for germination. In addition to hot water soaks, scarification, and chemical treatments to improve germination, more sophisticated techniques have been developed. These include priming, encapsulation, and coating with fungicide. Priming is a technique used to imbibe seeds in a concentrated osmotic solution, thus allowing water uptake, or imbibition, at a reduced speed into the seed. The physiological processes of germination begin but do not proceed as quickly as they would with regular imbibition. The advantage of this process is that it can be halted, and the seeds may be removed from solution, dried, packaged, and shipped. When they are subsequently planted, they will germinate as usual, but in a shorter period of time. Germination tends to be more uniform with this pretreatment.
Encapsulated seeds are covered with a protective coating, making tiny seeds easier to handle. The coating may be of a polymer that can incorporate fertilizer and fungicide, if desired. Coating with a polymer provides a dust-free method of applying fungicides to seeds. Larger seeds may simply be dusted with a fungicide.
HYBRIDS VERSUS OPEN-POLLINATED
Seeds that are collected from the wild are most likely to have been pollinated by more than one source. They may have also been self-pollinated, unless there is some mechanism that prevents this. The resultant offspring from this type of pollination, open-pollination, will most likely vary in many characteristics. Controlled crosses are made to obtain offspring with specific desirable characteristics. They may have improved traits such as larger flowers or more flavorful fruit. Hybrids or [F.sub.1] hybrids are developed for specific traits, such as improved disease resistance or increased yield.
Seed storage is a critical component of viability. The viability of a seed determines whether it will germinate or not, given the proper environmental conditions. Viability is important whether you are purchasing seeds or collecting your own seeds for use later. It is also applicable if you use only a portion of the seeds you buy and wish to save the rest for planting later. In this case, store the seeds in a sealed, airtight container in a cool, dry location. The crisper drawer of a refrigerator works well for the most part. However, you need to protect your seeds from the ethylene gases given off by some fruits, or you will risk reducing the viability of your seeds.
In commercial seed-storage facilities, large, walk-in coolers or cool rooms are used. Seeds are often packed in vacuum-sealed aluminum packets and placed into plastic containers with tight-fitting lids. Moisture and heat are the two main factors that cause deterioration of seeds and subsequent loss in viability. When stored properly, seeds may remain viable for several months to years, depending on the species (Table 3-2).
CONTAINERS AND MATERIAL FOR STARTING SEEDS
Flats, peat pots, miniature hothouses, and paper pots are some of the containers available for starting seeds (Fig. 3-3). The latter are made from pressing pieces of newspaper into a special mold. The others may be purchased. Miniature hothouses feature a flat for starting seeds and a plastic dome cover to maintain high humidity. Some kits include special germination media. Otherwise, specially formulated media may be purchased or made.
Starting seeds outdoors is called direct-seeding. Vegetable gardens are often begun outdoors, where seeds are planted directly into the ground. Seeds of woody and herbaceous plants may also be started outdoors in nursery rows. However, manypeople want to get a jump on their vegetable crops and will either begin seeds indoors in late winter or will buy transplants. Many vegetables and flowers are handled this way. Regardless of whether you want to start seeds indoors or outdoors, there are some basic factors you should understand to make the entire process as successful as possible.
Preparing the Seedbed
To plant seeds directly into the ground outside, loosen the soil with a shovel by turning it over and breaking the clumps. You may prefer to use a rototiller for this purpose. You should conduct a soil test to determine the texture of your soil and to find out whether fertilizers are needed. See chapter 5 for a discussion of soil testing and for information on how to do your own. If needed, some fertilizers may be mixed in at this time. If soil amendments are to be used, they may also be mixed in at this time. Add organic matter such as compost, aged manure, peat moss, or shredded leaves and grass clippings if desired (Fig. 3-4).
If you have poor-draining soil you may wish to construct raised beds to aid in drainage. On the other hand, if the soil is very light, you may wish to garden in sunken beds with berms surrounding them to allow for flooding of the area to be watered.
Germination Requirements: Temperature
The soil temperature outside will determine when seeds will germinate. There are cool-season, warm-season, and heat-loving crops. Cool-season crops are capable of germinating in cooler soils. They grow best in cooler temperatures, too. Some cool-season crops include lettuce, spinach, and cole crops. See Table 3-3 for a list of crops and their optimal seasons.
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Some cool-season crops will develop flowers and set seed when temperatures become too warm for them. This is called bolting. The leaf shapes may even change.
There are several problems that may affect seedlings that are started outdoors. They are more susceptible to the weather, pests that inhabit the soil, and the fungal pathogen that causes damping off. Damping off can spread rapidly and is a particular problem when nonpasteurized soil or soilless medium is used. Damping off can kill a young plant in a few hours or a few days. The best control is prevention by pasteurizing the soil or medium before use. Cutworms cut the seedling stem soon after germination. They hide in the soil during the day and feed at night. Pesticides may control these pests, which will eventually pupate in the soil and emerge as moths. A chemical-free method to save the plants is to place cardboard or paper-cup collars around the seedlings at the time of transplanting.
Planting Depth and Spacing
Pay attention to seed planting specifications on seed packets. If you have collected your own seeds, the following rule of thumb may be used in determining planting depth: plant approximately two times the diameter of the seed. If seeds are planted too deep, they may germinate but will exhaust their food supplies before reaching the soil surface. This is typically a greater problem than seeds planted too shallow, because the latter seeds will still send out a radicle, or primary root, and anchor themselves into the ground. Radicles and secondary roots exhibit geotropism that causes them to grow downward, toward the center of the earth.
How a Seed Germinates
When seeds are first planted and watered, they begin the process of imbibition, soaking up water at a very fast rate. The first visible sign of germination is the radicle that emerges and quickly grows several inches deep into the soil. This event occurs in a matter of a few days to several months, depending on species. Once the seed is anchored and is able to take up nutrients from the soil, cotyledons appear. At this point, emergence is said to have occurred. These seed leaves are soon followed by development of the first true leaves. See Fig. 3-5 for an example of a germinating seed.
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Because many seed lots do not have 100% viability, it is a common practice to overseed. Once the seeds emerge, however, it is very important to remove excess seedlings so that each seedling has ample space for optimal growth. Seeds should be thinned to the appropriate spacing within 1 to 2 weeks after emergence.
STARTING SEEDS INDOORS
Seeds are started indoors to add length to the growing season. By starting seeds indoors under controlled, optimal conditions, plants may have 6 or more weeks of additional time for growth. The optimal conditions to provide, if you want to start seeds early, are proper lighting, temperature, and humidity. If you can control these three factors, you may produce healthy, vigorous seedlings for transplanting outdoors in 6 to 8 weeks.
When seeds are started indoors, flats or trays are usually used. In commercial greenhouse production, seeds are started in seedling trays that have small, individual cells for each seed. For example, a 12-inch by 24-inch tray will have 256 or 528 cells. These are called plug trays. Each seed is grown into a plug, with its own root ball. The entire seedling is easily removed and transplanted to a plug tray with larger cells about 2 weeks after germination. Special seed starter mix that has been designed for very good drainage is used. This factor is important because such shallow trays tend to have relatively poor drainage. Plug trays offer an advantage over broadcasting of seeds in a large flat in that each seedling has a discrete root system and can be fairly easily handled for transplanting. In a large flat, seedling roots grow together and are quite difficult and time-consuming to handle for transplanting.
Small-seeded annuals and perennials, whether they are flowers or vegetables, are generally started in flats or trays. Larger-seeded plants may also be started in soilless media in a variety of container types. For example, tree seedlings may be started in 4- to 10-inch-deep plastic or cardboard containers or sheaths. These may be specially purchased or made from household items such as milk cartons. Many tree seedlings send down a long taproot when they first germinate, so these containers allow room for that normal process. Some growers limit the depth of these containers to artificially overcome juvenility in trees such as oak, which may have a 6- to 10-year juvenility period. In this way they are able to produce acorn-bearing oak trees in approximately 3 years. Plastic containers for tree seeds may be designed with small furrows along their length. These help to orient the roots into a downward-growing position in an effort to prevent root-circling inside the container.
Because gravitational pull is stronger on a deeper container than on a shallow one, drainage is better with larger containers. When larger containers are used to start seeds, field soil may be added to the soilless medium mix. Shallow seedling trays require use of a special germination medium that is well drained and has a coarser grade of vermiculite to allow better drainage.
Soilless media are used for starting seeds indoors. When seeds are started indoors, it is important to use appropriate media for root development and growth. The primary way to control this is by providing proper drainage. Well-draining media will allow moisture to drain away from the roots and also provide pore sizes for adequate aeration. The primary components for soilless media are peat moss, perlite, and vermiculite.
Peat moss is an organic product that is harvested from peat bogs. It provides good moisture-holding, will slowly decompose to release nutrients, and resists compaction somewhat. Perlite and vermiculite are derived from mineral sources. Perlite is a volcanic mineral that has been heated to a high degree. In the heating process, the minerals expand. Perlite comes in a variety of sizes and aids in drainage in soilless media blends. Individual particles are easily visible to the naked eye, and so perlite is too large to satisfactorily hold nutrients or moisture. Vermiculite is a stratified clay that also has been expanded through heating. Upon watering, vermiculite absorbs moisture in its layers, expanding like an accordion. However, it does not hold the water tightly against the ability of plant roots to remove it for their use. It is an ideal moisture-retaining component in soilless media. Vermiculite is available in a variety of sizes. Because it is a clay, it does have the ability to compact, so must be used in moderation.
PASTEURIZATION. When you use field soil or mix your own soilless medium from raw materials, it is necessary to eliminate disease pathogens and insect eggs that could adversely affect your seedlings. A process has been developed to accomplish this called pasteurization. In this process, soil or soilless medium is moistened, heated to 140 to 160_F for 30 minutes, and then allowed to cool before using. This process will kill most weed seeds, plant pathogens, and insect eggs, while allowing beneficial microorganisms to remain intact. Beneficial organisms, such as mycorrhizae, are important for healthy plant growth (see box).
Greenhouse growers may have a special soil pasteurization unit or they may construct a device with a metal garbage can and steam or hot water. At home, you may use an old baking pan, moisten the soil or medium, cover it with tin foil, and heat in the oven. Use a soil thermometer or a dedicated meat thermometer to monitor the temperature of the soil or medium.
TEMPERATURE. During germination, seeds must remain moist, and the soil temperature must be maintained at the optimal temperature for the species. Plants will generally germinate at cooler or warmer temperatures, depending on species requirements. (See Table 3-3 for seed germination temperature requirements.) Several methods are used to provide proper soil temperature. Electric heating mats and hot water cables are two popular methods used in greenhouse production. Special cabinets may be built in which many seed flats can be placed. A heater is placed inside, along with a humidifier. Flats are left only long enough for germination to take place; then they are removed to a greenhouse bench under a mist system. When seeds are started at home, after germination plants may be maintained under high humidity using plastic dome covers for flats or plastic sheeting suspended over the plants. Benchtop systems with lighting and plastic sheeting are also available for purchase (Fig. 3-8).
Mycorrhizae Mycorrhizae are fungi that live in the soil and form symbiotic relationships in plant roots (myco- fungus, -rhizae roots). The relationship is beneficial to the plant because mycorrhizae help take up nutrients from the soil. The mycorrhizae benefit because the plant provides them with sugar. There are many species of mycorrhizae, and they have specific relationships with plant species, such that a specific plant is host to a specific mycorrhiza (Figs. 3-6 and 3-7). Plants are usually infected with their particular fungus in their native soil. Some plants do not thrive at all if they are grown in non-native soil. Mycorrhizae aid plants in several processes, including nutrient and water uptake and root and plant growth. They also improve yield and disease resistance and reduce transplant shock and drought stress. There is even evidence that they control parasitic nematodes that would be harmful to plants. They do this by forming loops with their filaments that the nematode eventually passes through. The filament then tightens around the nematode as it moves and strangles it. The extent of interactions of mycorrhizae with plants and the rhizosphere is certainly still unfolding, but they clearly play a crucial role in the health and longevity of plants, and perhaps their very survival as species.
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HUMIDITY. High humidity, 90% to 100%, is optimal for seed germination of most species. For this reason, seed flats may be covered with plastic film, clear plastic dome lids, or even glass plates. In a greenhouse, plants are placed on a mist bench that is designed to maintain high humidity. Moisture sensors have been specially designed to turn the mist system on and off (Fig. 3-9). If you start seed in a small flower pot, you may use a plastic bag as a covering to maintain humidity.
LIGHTS. Seeds are usually covered after planting, but some species actually require light for germination. Other species require absolute darkness, and some seeds will germinate either way. Table 3-4 lists plants that require light for germination, and Table 3-5 lists plants that require dark.
If seedlings are not provided with adequate light, they may stretch and become thin and lanky, a phenomenon known as etiolation. In addition, they will lack sufficient chlorophyll. Etiolation also results when there is a high ratio of red light, such as that provided by the typical household light bulb or incandescent lamp. Plants that are crowded too closely together will become etiolated, and the resultant lanky growth is impossible to remedy. Etiolation may be avoided by providing adequate lighting with an appropriate light source and spacing plants to provide ample room for growth. Optimal light can be provided in a sunny window, using specially designed grow-lamps or fluorescent lamps.
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Plants usually respond best to transplanting when they are about 6 weeks old. At this time, they should have a well-developed root system but not be root-bound. They should have several expanded leaves and continuous new growth. Before they are planted directly into the garden, plants should be hardened off. This is a process of slowly introducing them to outdoor conditions while they are still in their containers. Hardening off (also called conditioning) can be achieved by placing the seedlings outdoors during the warmer part of the day, for 1 or 2 hours to start and increasing the amount of time over a period of 1 week to 10 days. If weather conditions permit, seedlings may be left out for 24 hours for 1 or 2 days before the planned planting date. Of course, if freezing temperatures are predicted, then the plants should not be placed outside. If freezing temperatures occur after transplants have been moved to the garden, it will be necessary to protect them from frost. They should be covered with a sheet, plastic, or a special nonwoven row cover fabric designed for this purpose. Be sure to remove plastic during the day, as it can warm up too much, especially on a sunny day. Use a medium-to-heavy gauge wire to create hoops to hold the covering off the plants.
Several methods are available for warming the soil and protecting transplants early in the season. Some of these methods are season extenders, allowing planting to the garden 4 to 6 weeks earlier than usual. They include the use of hot caps, Wall O' Water products, cold frames, and plastic mulch for heating the soil. The first three protect the plant from cold temperatures, whereas the latter one warms the soil earlier. (See chapter 13 for more detail about plastic mulch and the colors to use.)
STARTING FERNS FROM SPORES
Ferns are shade-loving plants with attractive foliage. Many gardeners like to include ferns in shady woodland gardens. However, not many people are aware of the reproductive process of ferns, nor the fact that they could start their own ferns from spores. Although ferns require a long time to develop with this process (up to 6 months), the outcome is very rewarding.
Ferns reproduce by spores rather than flowers. The fronds bear the spores, often on their undersides or on a special frond-like structure (Fig. 3-10). Collect the spores by tapping or scraping the spore-bearing sori (singular sorus) into an envelope, and allow them to dry for 1 day or so. The spores will be released within 1 day into the envelope and will be ready to distribute onto a growing medium.
Sprinkle the spores on a well-wetted medium in small pots (peat pots work well) or a flat and place under a misting system or cover with plastic dome lids to maintain high humidity. Make sure the spores do not dry out during the germination process. Spores will take 2 to 6 weeks to germinate, but when they finally do, you will observe the green, somewhat slimy-looking, prothalli. These are reproductive structures bearing male and female organs that will fertilize each other. A moist environment is required for the sperm to travel from the male to fertilize the egg, so be sure to maintain high humidity and mist regularly. The fertilized egg will slowly develop into a frond, and eventually your new fern will be recognizable. Be patient, because the entire process can take as long as 6 months or more. Each spore may develop into a new plant, so be ready to transplant ferns as they appear and to provide plenty of room for them to grow (Fig. 3-11).
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Multiple factors are responsible for the successful germination of seeds. A thorough understanding of all of these permits one to develop his or her green thumb. When buying seeds, purchase good quality seeds from a reputable source. When collecting seeds from the wild, you may need to apply treatments before germination can commence. Hybrid and open-pollinated seeds each have their advantages.
Seeds may be started in a variety of containers designed specifically for this purpose. Proper soil temperature and high humidity aid in high rates of germination in a timely manner. Direct-seeding in a well-prepared seed bed is practiced with many flowers, vegetables, and woody plants. Varying soil temperatures determine the season that certain plants may successfully germinate in the field.
The first event in germination is imbibition, or the uptake of water. This is followed by radicle emergence and then shoot emergence.
* Research the stratification requirements of various woody and herbaceous perennial plants. Are there differences in the amounts of time required at cold conditions? Group species together by the length of stratification required.
* Dissect seeds and locate the embryo and endosperm. Draw a diagram illustrating these parts on different types of seeds.
* Germinate seeds using the paper towel/rag-doll method or a Petri dish. Compare germination rates of fresh seeds and older seeds.
* Grow an avocado from a seed.
* Find the sori and spores on a fern. Collect some of the spores in an envelope.
1. Name the parts of a monocot seedling.
2. Name the parts of a dicot seedling.
3. If you have collected some herbaceous perennial seeds from the wild and you are not sure what they are called, how deep will you plant them if they are 1/8 inch thick (assuming they do not require light for germination). How deep will you plant them if they are 1/4 inch thick?
4. Describe the process of seed germination, beginning with imbibition.
5. What is stratification and how does it work?
6. What is scarification and how does it work?
7. What are some of the ways you can ensure you are buying good quality seed?
8. Describe the process of pasteurization and discuss why it is used.
9. What are peat moss, perlite, and vermiculite? Where do they come from? How and why are they used?
10. Why is special rooting media recommended with use of shallow containers? Why are some seeds started in deeper containers?
Bubel, N. (1988). The new seed starter's handbook. Emmaus, PA: Rodale Books.
Cutler, K. D. (Ed.) (1998). Starting from seed: the natural gardener's guide to propagating plants. Brooklyn, NY: Brooklyn Botanic Garden.
Dirr, M. A., & Heuser, C. W. (1987). The reference manual of woody plant propagation: from seed to tissue culture. Atlanta, GA: Varsity Press.
Picking the Best
Many choices of cultivars are available for most garden plants. How do you know which ones are better? How do you decide what to grow? There are several organizations whose aim it is to help you do exactly that. The best ones operate by conducting trials with new cultivars in numerous locations and comparing the new against older, proven cultivars. All-America Selections (AAS) is an organization whose mission it is to "promote new garden seed varieties with superior garden performance judged in impartial trials in North America." Trials are conducted at universities, seed companies, or at other horticultural institutions. A judge at each location rates both flowers and vegetables for qualities such as length of flowering season, disease or pest tolerances or resistance, and fragrance; fruit quality, earliness to harvest, total yield, fruit taste, and ease of harvest. Cultivars are evaluated throughout the season and receive an overall score. Only the entries with the highest average scores are considered to be worthy of an AAS Award. Some past winners of AAS Awards include 'Gypsy Deep Rose' Baby's Breath, 'Sweet Beauty' Watermelon, 'Papaya Pear' Summer Squash, and 'Purple Majesty' Ornamental Millet.
Dr. Marietta Loehrlein currently teaches horticulture classes at Western Illinois University in Macomb, Illinois. She earned both her bachelor's degree in Agronomy and her master's degree in Plant Genetics at The University of Arizona. Her master's research project was concerned with germination problems associated with triploid seeds, from which seedless watermelons grow. Following that she worked for 5 years in a breeding and research program for Sunworld, International near Bakersfield, California. She worked with peaches, nectarines, plums, apricots, and cherries. Then she returned to school to earn her Ph.D. in Horticultural Genetics at The Pennsylvania State University. Her Ph.D. research focused on flowering processes in regal pelargonium (also called Martha Washington geraniums). While at The Pennsylvania State University, she bred a new cultivar of regal pelargonium, "Camelot." At Western Illinois University, Dr. Loehrlein teaches nine courses: Greenhouse and Nursery Management, Introductory Horticulture, Landscape Design, Landscape Management, Home Horticulture, Plant Propagation, Turf Management, and two courses in Plant Identification.
TABLE 3-1 Stratification Requirements of Some Perennial Plants NAME SPECIES REQUIREMENT Woody plants American chestnut Castanea dentata Fall plant or 2-4 mo 41[degrees]F Apple Malus spp. 1-3 mo at 32-50[degrees]F Colorado spruce Picea pungens 1-2 mo 34-41[degrees]F * Common lilac Syringa vulgaris 2 mo 41[degrees]F Crabapple Malus spp. 1-4 mo 41[degrees]F European mountainash Sorbus aucuparia 2-4 mo 41[degrees]F Flowering dogwood Cornus florida Fall planting or 3-4 mo at 41[degrees]F Maple Acer spp. 2-4 mo 41[degrees]F Mockorange Philadelphus 20-40 days 32-39[degrees]F coronarius Red oak Quercus rubra Fall planting or 2-3 mo 41[degrees]F Peach Prunus persica 2-3 mo 41[degrees]F Spruce Picea spp. Fall planting or 1-2 mo 41[degrees]F Redbud Cercis canadensis 3 wk at 32-40[degrees]F Herbaceous plants Butterfly milkweed Asclepias tuberosa Older seeds only: 1-2 wk 36-40[degrees]F Clematis Clematis 2-6 mo 40[degrees]F Columbine Aquilegia 2 mo 34-40[degrees]F Daylily Hemerocallis 6 wk 34-36[degrees]F Liatris, gayfeather Liatris spp. 6-10 wk 40[degrees]F Penstemon, Penstemon spp. 8 wk 59[degrees]F beardtongue * Stratification not required, but results in uniform germination and/or reduced time to germinate. TABLE 3-2 Seed Viability if Stored at Optimal Conditions NAME GENUS AND SPECIES YEARS OF VIABILITY American elm seeds Ulmus americana <1 Beet Beta vulgaris 5-10 Carrot Daucus carota 5-10 Corn Zea mays 2 Cottonwood Populus deltoides <1 Lettuce Lactuca sativa 5-10 Lotus Lotus spp. >50 Onion Allium cepa 2 Silver maple Acer saccharinum <1 Squash Cucurbita pepo 5-10 Watermelon Citrillus lanatus 5-10 Wheat Triticum aestivum 30 TABLE 3-3 Cool-Season/Warm-Season/Heat-Loving Crops SOIL CROP GENUS AND SPECIES SEASON TEMPERATURE Broccoli, cabbage, Brassica oleracea Cool 55-65[degrees]F cauliflower Beans Phaseolus spp. Warm 65-75[degrees]F Carrot Daucus carota Cool 45-55[degrees]F Corn, sweet Zea mays Warm 50-60[degrees]F Lettuce Lactuca sativa Cool 40-65[degrees]F Onion Allium cepa Cool 40-50[degrees]F Peas Pisum sativa Cool 45-65[degrees]F Peppers Capsicum annuum Warm 65-75[degrees]F Pumpkin Cucurbita maxima, Warm 60-70[degrees]F Cucurbita moschata, Cucurbita pepo Radish Raphanus sativus Cool 40-65[degrees]F Spinach Spinacea oleracea Cool 55-65[degrees]F Tomatoes Lycopersicum Warm 65-75[degrees]F esculentum Watermelon Citrillus lanatus Hot 70-80[degrees]F Zucchini Cucurbita pepo Warm 65-75[degrees]F TABLE 3-4 Plants That Require Light for Germination COMMON NAME SPECIES Ageratum Ageratum houstonianum Begonia Begonia spp. Blanket flower Gaillardia x grandiflora Carrot Daucus carota Coleus Coleus x hybridus Columbine Aquilegia spp. Coreopsis Coreopsis grandiflora Impatiens Impatiens wallerana Lettuce Lactuca sativa Lobelia Lobelia erinus Oriental poppy Papaver orientalis Pentas Pentas lanceolata Petunia Petunia x hybrida Primula Primula spp. Salvia Salvia splendens Shasta daisy Chrysanthemum x maximum Snapdragon Antirrhinum majus Sweet alyssum Lobularia maritima TABLE 3-5 Plants That Require Dark for Germination COMMON NAME SPECIES Bachelor's buttons Centaurea cyanus Borage Borago officinalis Chinese primrose Primula sinensis Coriander Coriandrum sativum Larkspur Delphinium elatum Nemesia Nemesia strumosa Pansy Viola x wittrockiana Phlox Phlox spp. Poppy Papaver spp. Portulaca Portulaca spp. Pot marigold Calendula officinalis Sage Salvia x sylvestris Sweet pea Lathyrus odoratus Treasure flower Gazania regens Verbena Verbena spp. Vinca Catharanthus roseus
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|Author:||Loehrlein, Marietta M.|
|Publication:||Home Horticulture: Principles and Practices|
|Date:||Jan 1, 2008|
|Previous Article:||Chapter 2: reproduction in plants: the birds and the bees, fruits, and seeds.|
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