GARDENING : OVERDUE PRAISE FOR PENSTEMON.

Byline: Joshua Siskin

The time has come to praise the Penstemon.

Actually, the time to praise the Penstemon is long overdue, since no other plant is more appropriate for the Los Angeles garden. It grows in the sun and blooms most of the year. Its trumpet flowers come in purple, pink, red, lavender, salmon, peach and white. It grows from 6 inches to 4 feet tall, depending on the species. Penstemon self-sows and propagates effortlessly from cuttings. Some of the most spectacular Penstemons, including the lavender Penstemon spectabilis, are California natives. Another bonus: Penstemons make excellent cut flowers for vase arrangements.

The Penstemon is related to the snapdragon and to the foxglove, with its flowers larger than the former and a bit smaller than the latter. You wonder why Penstemon isn't displayed more prominently in nurseries; perhaps because, once you discover its charm and versatility, you won't need to buy any more flowers for the sunny bed or border.

While individual Penstemon plants probably won't live longer than a few years, their seedlings come up as volunteers and they can naturalize (take over a large area), of the garden. In addition, their shoots have a tendency to develop above-ground, adventitious roots, which are easily detached and layered in the ground or potted, to be grown into more new plants.

One warning is in order: Do not grow Penstemon where soil drainage is less than perfect. To make sure soil drains properly, take the counsel of the ``Sunset Western Garden Book'' literally. ``Best in loose, gravelly soil,'' opines Sunset. Thus, when planting Penstemon, work plenty of gravel into the earth, or at least build a raised, fast-draining bed for your beard tongue (Penstemon's nonsensical common name).

Two months ago, some students of mine planted about a dozen Penstemon that had been growing in 1-gallon containers. Pea gravel was used both as soil amendment and as mulch. Today, each plant is laden with flowers. Initially, watering was done three times a week. Now that the plants have established themselves, a single weekly soaking is adequate. Some reports indicate that once or twice a month watering may be enough to maintain year-old plantings of Penstemon.

In the students' planter bed, there are two equally water-thrifty species that make excellent companions to Penstemon from both a design and a cultural perspective. A yellow-flowered yarrow, Achillea ``Moonshine,'' and a purple verbena ground cover are growing in parallel rows outside the Penstemon. Both the yarrow and the verbena have rough-textured yet lacy leaves that contrast well with the smooth, shiny, spear-shaped foliage of Penstemon. The gray leaves of Achillea also do an excellent job of highlighting the colors of the surrounding flowers.

As an aside, the most powerful effect the gray-leaved achillea can achieve is when it is planted in combination with Aeonium arboreum ``Zwartkop,'' a rosetted succulent plant with a one-of-a-kind dark burgundy color.

Speaking of dark burgundy, there is a remarkable shade-loving plant that you should know about and, no doubt, some of you already do know about. It looks to all the world like a large oxalis with triangular, bisected, trifoliate leaves that are burgundy in color, or maybe I should say dark violet with a dash of maroon. It produces pale lilac flowers. If anyone knows the scientific name of this species, please advise!

Another question for readers: The other day, at a local nursery, I was sampling lettuces, growing in six packs, with one of the salesmen. We both agreed that all - from buttercrunch to red leaf varieties - were on the bitter side. Can anyone recommend a less bitter lettuce variety for L.A. gardens? Or perhaps there's a cultural practice that gives lettuce a milder taste. Unfortunately, the popular, crunchy, mild ``Iceberg'' lettuce is difficult to grow in our city.

Tip of the Week: Consider cannas for half-day sun locations. There are dozens of varieties of this perennial whose flowers look like irises or ginger lilies growing out of clumps of banana leaves. The challenge is to keep the flimsy canna leaves from crisping, a result of overexposure to hot sun and dry air. Somehow, some way, enhance humidity around canna leaves to keep them fresh and green.