IN THE GARDEN IN PRAISE OF PLANTS THAT MAKE THEIR OWN FERTILIZER.

Byline: JOSHUA SISKIN

As if their fragrant blue flowers were not enough of an attraction, California lilacs (Ceanothus) have another attribute that makes them a sure choice for Valley slopes and gardens: the ability to manufacture their own nitrate fertilizer, albeit with the assistance of soil-dwelling aerobic bacteria.

Nitrate is the most important ingredient in the mineral nutrition of plants. Virtually every fertilizer formulation you will find in a bag, box or bottle contains nitrate.

Legumes, including every type of pea and bean, as well as carob, acacia, mesquite, silk (Albizia) and coral (Erythrina) trees, are the most famous partners of nitrate-synthesizing bacteria. These bacteria inhabit the legume roots, forming bumps or nodules that are clearly visible when the plants are extracted from the earth. In return for the nitrate they provide, these beneficial bacteria receive plant carbohydrate in return.

Several California natives also make use of symbiotic, nodule-forming bacteria, with Ceanothus (See-a-NO-thus) being the most prominent among them. Any day now, you will see the pale blue and white inflorescences of this magnificent shrub begin to show themselves on the slopes and in the canyons around Los Angeles. If you are hiking and encounter Ceanothus, rub its flowers between your palms and you will appreciate its cleansing, soaplike properties.

Although the flowers of local Ceanothus natives are invariably white or pale blue, be aware that almost every shade of blue is found in one Ceanothus variety or another. Hybrids of this genus abound, and you can find azure and marine blue Ceanothus cultivars in nurseries. The spectrum of Ceanothus species and hybrids includes tree, shrub and ground-cover forms.

The key to growing healthy Ceanothus plants is to leave them alone. They are best planted now, in the fall, and should occasionally be soaked to get them on their way, but one year after planting all watering should cease. Fertilization is not required and could damage the beneficial soil- dwelling bacteria on which Ceanothus rely. Pruning should be done with great reluctance, if at all, since this procedure encourages entrance of disease organisms through cut stems.

The rap against Ceanothus plants is that they are short-lived. Yet in their habitat they often thrive for 50 years or more. In gardens where watering is excessive, however, they seldom live more than a few years. If you see Ceanothus leaves turning yellow, either you are watering too much, or the soil is too heavy to drain away whatever water is being applied.

Several other attractive California natives rely on the services of symbiotic bacteria as their primary source of nitrate.

The coffeeberry (Rhamnus californica) is a shrub that grows into a dense hedge with red or black berries, attracting birds and other wildlife. It grows to 6 feet tall or more and may be shaped into a hedge. The silver-leafed buffaloberry (Shepherdia) is also a magnet for birds on account of its orange berries, which can also be made into jelly and jam.

White alder (Alnus rhombifolia) is one of the most magnificent local native trees, and it, too, relies on subterranean bacteria for a steady supply of nitrate. Unlike the long list of drought-tolerant natives, however, white alder is riparian, which means it is found along riverbanks or arroyos and needs regular water to look its best. When well-watered, there is no more beautiful tree, with its pyramidal shape and lustrous foliage. Avoid the mistake of depriving it of water or relying on superficial irrigation to meet its water needs. Deep soaking the white alder once a week is what it requires; beneath its outer foliage (known as the drip line), encircle it with a soaker hose for best results.

Silver Italian buckthorn (Rhamnus alaternus 'Argenteo-variegata') is a Ceanothus relative native to the dry Mediterranean climate that mimics our own. This is one of the most decorative plants you could select, either to be trained as a hedge or left alone to grow into a solitary specimen tree. It has cream and green leaves along cinnamon-colored stems, and cuttings from it are highly prized as background or filler in flower arrangements. In the garden, it grows up to 8 feet tall, may be used in sun or partial shade and, once established, does not require irrigation.

TIP OF THE WEEK: After digging in your daffodil bulbs, plant Coreopsis rosea 'Sweet Dreams' in the soil above. This is a ground cover that produces pink and white daisy flowers for several consecutive months. Daffodils will bloom through it next spring, their yellow and orange colors contrasting nicely with the rosy Coreopsis.