IN THE GARDEN NEW GARDENERS, NEW PLANTS.

Byline: JOSHUA SISKIN

Not until they reach middle age, or later, do most people develop an interest in gardening.

For the first half of their lives, most simply lack the patience required to stop and smell the roses, much less take care of them. But as people age and begin to stay home more and go out less, they develop a natural kinship with the sedentary plants around them and so, in the end, become gardeners.

Today, as baby boomers advance into middle age and beyond, it is a great time to be a gardener. The number of new plant varieties available increases exponentially every year, and it makes sense for nurseries to grow these new varieties to meet the needs of an expanding population of gardeners. Also, in a culture of visual bombardment, there is a never-ending demand for what is different, exotic or new-looking.

Many new plant varieties attract attention not, as you might think, because of their flowers but because of their foliage. A case in point is the new 'Pink Marble' photinia. Photinia has long been a standout among shrubs because of one peculiar characteristic: Its leaves, when first formed, are a bright scarlet red - before they eventually turn to a prosaic green.

The 'Pink Marble' photinia variety, however, has leaves that first appear red and pink, then change to pink and green - and finally transmute into green foliage that is edged and splashed with white.

Another noteworthy new shrub is Berberis 'Bonanza Gold.' Berberis or Japanese barberry is primarily known for its burgundy foliage varieties. But now 'Bonanza Gold,' with chartreuse leaves, has also taken the stage. It is a semi-dwarf that grows to a height of 3 feet.

A new Japanese maple, Acer palmatum dissectum 'Palmatifidium,' is destined to reach exalted status in the pantheon of small trees. With the descriptive common name of ``weeping eagle's claw maple,'' its foliage is broader, stronger and more pendant than that of other Japanese maples. Leaf color is a shimmering, iridescent green, and tree height will not exceed 15 feet.

The other day, I was at the nursery and - as a double victim of love at first sight - had to pick up two plants that I had never seen before. One was Gaura 'Siskiyou Pink,' a spreading, drought-tolerant perennial with burgundy foliage and wispy pink flowers. All varieties of Gaura (rhymes with Laura) are known for their self-sowing prowess and will obligingly cover any patch of well-drained soil in the course of time. I also had to take home a Phygelius 'Pink Elf' whose hanging tubular flowers bloom for months on end. I had grown the common Phygelius, or cape fuchsia, with reddish-orange flowers before and could not resist its cousin that bloomed in pink.

Another plant that I was introduced to recently was a lavender-flowered Epidendrum orchid. Epidendrums are easily the toughest of all orchid types. They bloom practically year-round and are especially suited to container growing. By chance, I happened upon several large tubs of lavender epidendrums on a partially shaded patio in Sherman Oaks.

In more than 20 years of plant-watching in the Valley, I had only seen red, orange and yellow epidendrums and was pleasantly surprised at the sight of these lavender beauties. How do you care for epidendrums? The general recommendation is to regularly feed them with diluted, high nitrogen liquid fertilizer in spring and summer, but I have seen plenty of Epidendrums thrive on virtual neglect.

Be on the lookout for many new varieties of long-flowering, low- maintenance shrub and ground-cover roses. 'Panda Meidiland' is a white ground-cover rose with deep green foliage that flowers from spring to fall. 'Acadia' is a compact floribunda, growing to only 2 feet in height, with bronzish-orange blooms.