BE PREPARED FOR POLLEN.

Byline: JOSHUA SISKIN

As if gardeners did not have enough to worry about ... weeds, insect pests, diseases caused by fungi, bacteria, viruses, snails and slugs, gophers, squirrels, deer, sloping terrain, hard soil, bad weather - all of these make it challenging enough to be a gardener.

But what do you do when the plants themselves turn against you?

It is estimated that 20 percent of the population experiences some sort of allergic or asthmatic reaction to plants, and the percentage is rising. Usually, the problem is pollen. Pollen and mold spores have a cumulatively damaging effect as they settle in the bronchial and respiratory tracts.

The list of allergenic-pollen producing trees is long, and it includes many of the trees commonly used in Valley landscapes. Among these are ash, elm, poplar, willow, cedar, cypress, fruitless mulberry, podocarpus, birch, melaleuca, bottlebrush, sycamore, oak, silver maple, liquidambar, birch, olive, walnut, acacia, and Brazilian and California pepper trees.

Many of the above are dioecious species, which means they consist of separate male and female trees. In such cases, it is the male type only that causes a problem, since pollen is borne on male flowers. In dioecious species, seeds and fruit are borne exclusively on female trees. For instance, if you have a Brazilian or California pepper tree that produces small pink or red berry-like fruit, you have nothing to worry about pollen- wise since your tree is a female. But most deciduous and forest trees, such as birch, sycamore and oak, have both female flowers and pollen-bearing catkins on every tree, and are thus uniformly allergenic.

If you are looking for trees that do not carry allergenic pollen, consider ficus species (although their invasive roots are another problem), pines (which often become too big for an average-size yard) and crepe myrtles. In general, ornamental trees with showy flowers and fruit trees - such as jacaranda, southern magnolia, flowering pear and peach, apple, lemon and orange - are not allergenic. For a complete list of plants, rated by potency of allergen, go to the Web site at www.allegra.com and click on ``allergy-free gardening.''

The whole subject of allergenic trees and plants is an argument in favor of flower gardens. The gorgeous flowers that light up a garden are bee-pollinated; their pollen is sticky, relatively heavy and does not blow around in the air like the wind-borne pollen from shade trees and grasses. It is generally true that the less showy the flower, the more likely it is to be allergenic. Xylosma and boxwood shrubs, for example, whose flowers are virtually invisible, are highly allergenic. Grasses have flower tassels and spikes that are colorless and produce wind-blown, allergenic pollen. A drought-tolerant landscape that relies heavily on ornamental, prairie and California native grasses is likely to activate your pollen allergies in a big way. Finally, heavily perfumed plants such as star jasmine (Trachelospermum jasminoides) can cause allergic reactions in some people.

Aside from pollen, mold spores are the most allergenic denizens of the garden. Areas that are in heavy shade and never dry out, compost piles and woody mulches are all havens for mold spores. To reduce mold spores or their effects, prune out trees to let in more light, wear a dust mask when you turn your compost pile, and use stones or gravel as mulch instead of wood chips or shavings.

There is a trend in garden and landscape design to minimize the presence of allergenic plants. Yet there is a problem here. Already, the eco-police have outlawed many plants from the garden based on water requirement, combustibility or weediness. Take away allergenic plants as well and you begin to wonder what species, if any, will still be suitable for garden use.

TIP OF THE WEEK: If you find it hard to be outdoors because of your allergies, you will be best off working in the garden either early in the morning or late in the day toward evening. At these times, pollen count in the atmosphere is lowest.