Special Collections : About 3,000 Types of Succulents Are Housed in the Desert Garden Conservatory at Huntington Botanical Gardens

OF THE NEARLY 6,000 kinds of cacti and other succulents in the Desert Garden collection at the Huntington Botanical Gardens in San Marino, about half are missed by visitors. Not only is the Desert Garden Conservatory, which houses those 3,000 succulent plants, off the beaten path, but it also keeps odd hours. The succulents in the conservatory, especially the cacti, are a remarkable bunch. They are not tough enough to grow outdoors: Curator John Trager says roughly two-thirds of the world’s 10,000 or so succulent plants will not tolerate the climate in Southern California because they cannot take winter rain or cold or because they are simply too small to compete with larger varieties.

Several collections are worthy of note. The melocacti, many native to the Caribbean, look as if they are wearing hats. The growth on the top is actually a cephalium , which produces flowers. It takes about five years for the otherwise ordinary-looking cactus underneath to grow its flowering “hat,” and from then on only the cephalium grows taller.

Also look for the collection of mammillaria, the second-largest genus in the cactus family, which includes some of the best flowering specimens. The clumping rebutias, which are wonderful earthy colors in any season, are spectacular in spring, when they are smothered with flowers that are neon in intensity.

The setting at the conservatory is as remarkable as the cacti: All the succulents grow in the soft light of exotic Belgian hammered glass (though plebeian shower-door glass has been used to replace broken panes in a pinch), under cloth that provides 47% shade. In this pampered environment, the plants never experience temperatures colder than about 45 degrees, though in the summer temperatures are allowed to climb to about 100. For this reason, and because many plants are in flower now, spring is the best time to visit.

Unlike most glass houses, this one is on the dry side. A concrete floor carries off excess moisture, and the humidity is kept around 15% in winter and about 35% in summer. The plants are watered once a week in summer and only every two weeks in winter, when many are dormant.

It was the inspired idea of curator emeritus Myron Kimnack to house the collection in specially made unglazed stoneware pots. Each plant now sits like a jewel in an earthen setting. The first batch of pots was a light tan, but pots now end up with a touch more red, which emulates the reddish-tan soil common in desert areas. Many are shallow, but deeper ones are now used, Trager says, because they drain excess water faster.

The soil mix is two parts agricultural pumice (the 1/8- to 1/4-inch size), two parts composted organic matter (such as fir shavings and oak-leaf mold) and one part builder’s sand, though sand is optional (it stores more moisture than the coarser materials and is used for plants that want more water). A special system fertilizes plants with each watering.

Open Tuesday through Sunday , 2 to 4 p.m. Reservations required Sunday; (818) 405-2273.