SO MANY TEMPTING trees to choose from, and so little space. Cass Turnbull, landscape gardener and founder of Plant Amnesty (an organization dedicated to promoting better pruning) has seen too many overgrown trees planted in the wrong place. "Even small trees grow up," she says, "and they always get bigger than you think, sooner than you think!"

The top layer of a garden is the most important to plan and plant first, as it sets the scale for the plantings to follow. Trees in all their leafy beauty provide the garden with shade, depth, texture, color, bloom and berries, as well as creating a haven for birds and other wildlife.

Walking through a forest, you appreciate nature's design as plants grow in finely textured tiers, with tall firs towering over understory trees such as vine maples, skirted by the third level of shrubs and ferns. A medley of ground covers with its overlapping textures and colors scramble at the feet of the larger plants.

You can create the same feel of natural abundance, albeit on a smaller scale, in your own garden. And it all starts with planting wisely chosen trees.

The Northwest Horticultural Society plant sale, to be held next weekend, will feature interesting trees for smaller gardens.

"Fall is an excellent time to plant, as trees will have a chance to become established by winter, taking advantage of cool weather and fall rains. In effect you gain a whole year of growth, as roots continue to develop as long as the ground is not frozen, even while the tree is dormant," says Doug Bayley, a landscape designer and head of the plant sale.

It is all too easy for a gardener to feel like a kid in a candy store while perusing plant-sale tables and isles filled to overflowing with unusual, sumptuous foliage. Pacific Northwest plant lovers, blessed with what is arguably the best gardening climate in the world, are faced with an amazing and often overwhelming assortment of plants from which to choose. Here's some criteria to keep in mind when shopping for that most important and permanent layer of your garden:

1. Keep eventual size and width in mind. That cute little tree will grow up. The good news is that there are many beautiful trees that will top out at 20 to 30 feet.

2. Each tree granted precious garden space should have more than a burst of bloom in the spring or a flash of fall color to recommend it. The best have three seasons of interest, with flower, fruit, fall color and interesting bark or silhouette in the winter.

3. Reduce maintenance by choosing trees that are easy to care for. Many varieties are pest and disease-resistant, and grow to a lovely natural shape with little pruning. Make sure the tree will thrive in the soil and exposure your garden provides; it's the best way to ensure long-term health.

4. Siting deserves consideration. Will you look down at the tree? If so, the Silk Tree (Albizia julibrissin) with its fern-like foliage and spread of fluffy bloom appears as a pink cloud when seen from above, while the Japanese Snowbell Tree (Styrax japonicus) is perfect sited next to a patio so that you can look up into its delicate, white June-blooming bells dangling along tiered branches.

5. Finally, consider how a tree will mingle with its surrounding plantings. Some trees cast too dense a shade, or create such a dry, root-clogged soil that it is hard to layer them into mixed borders, large pots or other planting schemes as part of an overall garden picture.

A spectacular tree that meets all of the above criteria is the Golden Locust (Robinia pseudoacacia `Frisia'). Commonly known in Europe but too rarely seen in Northwest gardens, this golden-foliaged beauty grows rapidly to an upright oval shape at about 25 to 30 feet. It is a very prunable tree, and its soft, ferny foliage "twitters in the softest breeze" according to Bayley, who grows it in his Capitol Hill front garden. New foliage emerges a bright yellowish-gold in the spring and maintains its striking color until autumn. Fragrant white flowers bloom in May and June. Golden leaved and variegated plants as well as purple flowers or foliage complement the Golden Locust in mixed borders.

Another tree ideal for smaller gardens and lovely in all seasons is the purple-leafed Eastern Redbud (Cercis canadensis `Forest Pansy'). Susie Marglin, an Arboretum Foundation member with a garden in Medina, has surrounded this beauty with hostas. Marglin describes its leaves as a "rich, deep purple which break at the same time in early spring as the pinky-magenta flowers. They stay a shiny, showy purple all summer, and continue to enlarge after the blossoms fade." Slow-growing with ruddy bark, red fall color and a lacy winter silhouette, `Forest Pansy' is a year-round asset.

Oversized leaves that smell like peanut butter and fruit that looks "as if it comes from another planet" have earned the Harlequin Glorybower (Clerodendrum trichotomum `fargesii') a starring place in Steven Antonow's West Seattle garden. The `fargesii' variety, which is native to China, is worth seeking out as it is hardier and smaller than most, growing to only about 10 feet. Its coloring is truly outstanding, with large, soft dark leaves and metallic turquoise fruits accented by pointed pink calyxes. The Harlequin Glorybower is best grown surrounded by plantings to hide its legginess and accent its spreading crown. Antonow prolongs the display by growing a pink-flowering clematis up through its dark foliage, creating a striking contrast of texture and color for nearly a month in mid-summer until the tree's own fragrant white blossoms take over in August.

While deciduous trees bring exciting seasonal change to the garden, many smaller evergreens have an important role to play, too. Familiar plants, such as camellias and even the ubiquitous Photinia look new and can be effective when pruned up as trees with plantings underneath, and perhaps laced with a vine for another season of bloom. Hardy eucalyptus, such as the variety Archeri, with its aromatic, rustling blue-gray foliage, add an unexpected and exotic feel to mixed plantings.

No need to be intimidated by the forest of choices. Just pick a tree you can live with.

Valerie Easton is a freelance writer and a librarian at the Elisabeth C. Miller Library at the Center for Urban Horticulture. Kort Duce is a Seattle Times photographer.

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The Northwest Horticultural Society Plant Sale is Friday, Sept. 23, 10 a.m. to 5 p.m. and Saturday, Sept. 24, 10 a.m. to 3 p.m. at the Center for Urban Horticulture, 3501 N.E. 41st St. For information: 527-1794.