Most cacti come from arid climates and our image of a cactus is precisely that of a plant adapted to growing in harsh conditions: a swollen stem that holds water, no leaves in order to reduce moisture loss, abundant spines to protect its valuable assets, etc. You might imagine that therefore you need to sow cactus seeds in sand and under conditions of burning sun and great aridity… but you’d be wrong.
In fact, in nature, not even one desert cactus seed in 100 germinates… and not even one seedling in 1,000 reaches maturity. Most years, there is no germination at all. The seeds will lay dormant for years if necessary, waiting for Mother Nature’s sign it is time to start growing: a good, thorough rain. And yes, it does rain in a desert… occasionally. Only then, when the soil is moist, they will finally germinate. Seedlings then struggle to grow as quickly as possible, a real battle against the elements, but if the soil dries out too quickly, they die. But from time to time, there’ll be a year where the soil remains moister than normal or when there are repeated rains. It is under those conditions that the next generation of cacti really manage to get a good start.
So, unless you want to have success with only one cactus seed out of 1,000, don’t treat cactus seeds like desert plants. Instead, coddle them! Sow and treat them like any other plant. Here’s how:
Sow in a small pot using pre-moistened seed mix and simply press the seeds into the soil. Cover with a dome or a transparent plastic bag to maintain high humidity. (I know: that sounds illogical for a cactus, but trust me, it works!) And place the pot in a warm, well-lit place (light is necessary for germination), but away from direct sunlight, or else it will get too hot inside its plastic bag shelter. A location under a fluorescent lamp is perfect. Germination can take from several days to several weeks.
Curiously even though most cacti have no leaves as adults, they will as seedlings. Each chubby seeding bears 2 cotyledons, just like any other plant, only much thicker. At the junction of the two cotyledons a bit of spiky growth will soon appear: the stem of the baby cactus.
As long the seedlings are under plastic, no watering should be necessary, although if the soil surface dries out, you could lightly spray the growing mix with water. After 2-3 months of growth, gradually remove the plastic bag or dome, opening or lifting it a bit more each day until the seedlings are used to fresh air.
When the seedlings are acclimated to indoor conditions, transplant each into its own small pot. This time, use a cactus potting soil. Start watering them like a typical houseplant (not quite like a desert cactus at this point), that is, when the soil is dry to the touch. Moderate lighting is also still the best choice at this stage.
It is only about a year after the seedlings have been growing on their own that you should begin treating them “like cactus”, placing them in full sun and letting the soil dry out thoroughly before watering again. Cactus are fairly slow growers, but some will be quite presentable by their 2nd year. How long before they bloom? Mammillarias or rebutias may reach blooming size in as little as 2 to 3 years, but most small cacti are 5 or 6 years away from blooming. As for candelabra and barrel types, figure 15 to 50 years… if indeed they ever bloom.
Obviously, there are exceptions to the sowing recommendations made above. Several opuntias from northern areas, for example, will require an extensive cold treatment before germinating, but in general, the above described process will work well with almost any desert cactus.
You can find packets of mixed cactus seeds in you local garden center and most of the bigger seed catalogs (Stokes Seeds, Thompson & Morgan, Park Seed, Halifax Seeds, Chiltern Seeds, etc.) also offer packs of mixed seeds. For a more extensive selection of cactus seeds, ideal for the cactophile, try Cactus Store or Mesa Garden.