Building backyard green.

How to position, thin, and prune trees and other plants to create working layers of life from the ground up into the blue.

In my three-quarter-acre backyard in spring and summer, sunlight feeds green leaves in layer upon layer from ground level to 70 feet in the air. The leaves in turn fuel 60 different species of trees and other plants and a variety of animals.

I'm one of the animals benefitting from these working layers of green. My body runs on oxygen--the exhaust gas of living leaves. My spirit, too, benefits--from seeing the beauty and elegance of natural systems at work.

I like how my backyard looks, how it works, and how it requires so little maintenance now that everything is in place. But some maintenance and a lot of original work--plus an understanding of the "rules" of green plants--are required to establish and maintain working layers of green like this. Fortunately, many of the rules have been written down, and a careful observer can see them operate around or near his or her own home.

This I knew 11 years ago: If the Virginia pines in my backyard had their druthers, they would hog the sunlight by forming a green "tent" of leaves above everything else. By catching most of the sunlight, the pines could monopolize the area's water and nutrients because not much could grow in their dense shade. Old age, insects fires, ice, and wind would eventually tear holes in the "pine tent" and other plants would invade. but when and how?

This I also knew: I could create a more diverse and interesting plant community if I mediated between the plants struggling for space in my backyard. This meant cutting out some of the pines and pruning some branches of the remaining trees to give space and light to other plants trying to survive in the shade. And it meant moving some plants from bad to good locations and introducing others that would flourish in the new conditions.

My basic tools were a shovel, a felling saw, an ax, a pruning saw, and some clippers. My work was guided by three simple rules:

RULE No. 1: There can be only one thing in one space at one time. RULE No. 2: Living plants expand to fill the space available.

When two or more plants have applied Rule No. 2 for awhile, they inevitably start to compete for the same space. Rule No. 1 then kicks in with its absolute dictum. You will notice that there are no lawyers in the plant world. That's because plant rules have no varying interpretations and no appeal process. The plant that wins the space isn't the one with the best lawyer; it's the one that uses the best strategy to occupy a particular space for a particular time.

In this column last issue, I wrote about various plant strategies and the approach to life that trees use--they tend to compete vertically with a keep-your-leaves-above-the-competition approach. As the more aggressive trees move their leafy struggles higher and higher, shaded space becomes available below for plants that can grow on the sun, water, and nutrients left over.

Plants that can grow in the shady, reduced circumstances beneath taller plants are called tolerant. Plants that must have full sunlight are called intolerant. From years of observation, foresters have ranked trees by their tolerance of competition for sunlight, water, and nutrients. The scale goes from very intolerant to intolerant, intermediate, tolerant, and very tolerant.

Tolerance is not constant. Plants are more tolerant of competition when they're young and when there's ample water and nutrients. Near the northern edge of its range, a normally tolerant species will require more light. North of its normal range, it may do well only on warmer, sun-exposed, south-facing slopes. At the southern edge of its normal range, a species may grow only on north-facing slopes or in deep, shady valleys where it's cooler and there is less shady valleys where it's cooler and there is less exposure to sunlight.

Intolerant trees like Virginia pine grow quickly in an effort to gain dominant positions in the sun. If they keep their leaves above other trees, they do fine. If other trees overtop them, kiss your intolerant trees goodbye. Intolerant trees "self-prune"--they kill their own lower branches (by shading) as they add new branches and leaves above.

Other trees and plants survive by tolerance. Sugar maple is a very tolerant species. Its seedlings will grow, slowly, in the dense shade of mature trees. If something such as a windstorm or cutting lets sunlight in, the seedlings will take offend grow faster, establishing a new maple forest. Tolerant trees like sugar maple, as they grow taller, retain their lower branches for a long time because their leaves will tolerate shading.

RULE No. 3: Those plants that must have full sun die in the shade.

Keep intolerant plants on top in the full sun; thin and prune to let some sun shine down to the tolerant plants closer to ground level.

GETTING SPACED OUT

Little plants become big plants. A one-year-old white-pine seedling that will fit in a teacup can eventually grow more than 100 feet tall and have branches and roots also reaching out close to 100 feet. That's why green plants require adjustments in spacing. Nature makes her adjustments by allowing countless numbers of plants to die to make way for the larger ones. Some of the plants that die in this process may be ones that you would like to have survive.

You can see if plants are squeezed for space by simple observation. Are the leaves of adjoining plants overlapping each other? Is growth slow? Are leaves and branches dying back from intense shading?

Here are some spacing guidelines for mature trees: Large trees, over 60 feet tall at maturity, should have 40 to 60 feet between them. Medium trees, 30 feet tall at maturity, need spacing of 35 feet. Trees that mature at less than 30 feet tall should be kept 25 feet apart. Try to keep large trees 30 feet away from road intersections and 15 feet from driveways and buildings.

Many of my trees are closer together than this because they are still young, but I will gradually thin toward the ideal spacings while keeping the spaces between the larger trees filled with small and medium-size tolerant trees such as hollies and dogwoods. At ground level, I'll add shade-tolerant ferns and various wildflowers.

There are some plants that simply are not suitable as close neighbors to others because they have learned to poison the competition through allelopathy--warfare in which one plant inhibits another through the release of chemicals. The chemicals are generally held in the foliage and fruits and are released in decomposition. Roots also sometimes exude the chemicals.

Some examples:

* Sugar maples release phenolics that inhibit yellow birch.

* Walnut trees give off juglone, inhibiting other nearby trees, shrubs, and herbs. Walnut leaves have even been used to repel fleas.

* Oaks exude courmarins and other phenolics, inhibiting herbs and grasses.

* Sumacs, rhododendrons, elderberries, and bracken ferns release phenolics that inhibit Douglas-fir. Bracken ferns are a factor in preventing reforestation of Douglas-fir areas and some other species as well.

* Asters and goldenrod exude phenolics that inhibit sugar maple and black cherry.

PLANNING YOUR GREENERY

1. Orient your plot--where is north, south, east, west? In North America, if you stand with your back to the sun at noon, you'll be facing north; east will be on your right side and west to your left. If your land slopes steeply north away from the sun, or south toward the sun, warmth and exposure to sunlight will be important factors to consider. Plants on the south side of a big tree will get more light than those on the north side. Observe the shadows at different times of day and different seasons; you will be working with sunlight coming from specific directions.

2. Get several guides to identify your local trees and plants. Look for guides that list the mature size of the plants and indicate such things as shade tolerance. The tolerance list in this article can help you here.

3. Survey what you already have. Are intolerant plants overtopping tolerants? Does your plot already contain mixtures of species? How large are the various plants now, and how large will they be at maturity? Are any of the plants allelopathic?

4. Consider: What would you like the area to look like at various times of year? How will you use the area? How does it fit with surrounding property?

5. Remember Rules 1, 2, and 3.

6. Start to thin, prune, plant, and transplant to get to your goal. If you are not familiar with safe tree-felling techniques, get some training or knowledgeable help. Large trees falling on you, your house, or the neighbor's house can be a bummer.

The next time we meet, I'll discuss how land and the process of growing green things get taxed and how we might better encourage the maintenance of vital green growing areas through new tax policies. We can hope that by July the ritual trumpeting and pawing at the ground will be done in Congress and our lawmakers will be ready to work on such matters. Bring your calculator or your Pentium chip--if it'll do math.

RELATED ARTICLE: QUESTIONS FOR YOUNG PEOPLE

(You'll find the right answers in "Green Side Up")

"TOLERANCE" IN TREES MEANS:

1) You can call them names and they don't care 2) They will live in the shade of other trees 3) They like any 1rind of music--rock, rap, country, or classical

RULE NO. 1 (to create working layers of green) IS: 1) Take a number for better service 2) There can be only one thing in one place at one time 3) No smoking

BLACK WALNUT TREES ARE:

1) Allelopathic and intolerant 2) Conifers 3) A new rock-music band

VIRGINIA PINES ARE:

1) Intolerant sun-hogs 2) Menthol cigarettes 3) A resort area in the Blue Ridge Mountains 

TREES THAT ARE 60 FEET TALL NEED:

1) Certificates for the Big and Tall Shop 2) Blinking red lights to warn aircraft 3) Spacing of 40 to 60 feet between them

RELATED ARTICLE: GETTING STARTED WITH WILDFLOWERS

There are some excellent guides to growing wildflowers and native plants. I like Taylor's Guide to Natural Gardening (Roger Holmes, editor; Houghton Mifflin Co., Boston, 1993), and The Complete Shade Gardener (George Schenk, Houghton Mifflin Co., 1984). Both books list sources of plants and have excellent color and black-and-white photos.

Public and private landowners frown on having wildflowers removed from their lands. To acquire wildflowers, use the books' sources, or the mail order firms listed here:

Arizona

Desert Enterprises P.O. Box 23 Morristown, AZ 85342 602/388-2488 Seeds--cacti, desert shrubs, grasses, trees, wildflowers; free catalog

California

Wildwood Nursery P.O. Box 1334 3975 Emerald Ave. Claremont, CA 91711 714/593-4093 Plants and seeds--grasses, shrubs, trees, wildflowers; catalog $1

Colorado

Neils Lunceford Box 2130 140 Blue River Pkwy. Silverthorne, CO 80498 303/468-0340 Plants and seeds--high-altitude grasses, shrubs, trees, and wildflowers; catalog and plant list free

Delaware

Winterthur Museum and Gardens Kennet Pile, Rt. 52 Winterthur, DE 19735 302/884-4600 Plants--shrubs, trees, and wildflowers; catalog $1

Illinois

The Wildflower Source P.O. Box 312 Fox Lake, IL 60020 Woodland plants (such as nine kinds of trilliums); catalog $1

Minnesota

Prairie Moon Nursery Rte 3, Box 163 Winona, MN 55981 507/452-1362 Plants and seeds--prairie, woodland, and wetland grasses and wildflowers; catalog $2

North Carolina 

Niche Gardens 1111 Dawson Rd. Chapel Hill, NC 27516 919/967-0078 Plants--wildflowers of the Southeast; catalog $3

North Carolina

Gardens of the Blue Ridge P.O. Box 10 Pineola, NC 28662 704/733-2417 Plants--great source for shade-loving wildflowers and ferns; catalog $2

Oregon

Forest Farm 990 Tetherow Rd. Williams, OR 97544 503/846-6963 Plants--shrubs, trees, wildflowers; catalog $2

Virginia

Andre Viette Farm and Nursery Rt. 1, Box 16 Fisherville, VA 22939 703/943-2315 Plants--wildflowers; catalog $2

Washington

Foliage Gardens 2003 128th Ave. SE Bellevue, WA 98005 206/747-2998 Plants--more than 100 hardy ferns; catalog $1