Growing Cacti from Seed
Use a separate small pot (2½" - 3") for each named packet, and about three such for a mixed packet. Yoghourt containers are very satisfactory, but holes must be made at the bottom. Cover the holes by a piece of cotton wool or glass fibre (to draw up water from underneath) and fill with any light crumbly free-draining and mildly acidic soil (such as, e.g., equal parts of sieved loam, sieved peat, genuine horticultural sand, and seed-grade perlite). Vermiculite should not be used unless pricking out is intended by the beginning of the second year of growth, because over the longer term the alkalinity inherent even in the so-called low-alkalinity grade will win over the acidity of the peat, and will kill the young plants. Do not add any fertilizer. Lightly firm the soil and level off about ½" below the rim of the pot, removing any large lumps from the surface.
Scatter the seed evenly, pressing any large ones into the surface. Do not bury the seed, it needs air!
Place the pots in a tray of water, about ¼" deep, and refill the tray as necessary to maintain some degree of continuous moisture at the soil surface.
Continuous warmth is essential for germination. A day temperature of 70-80°F (21-27°C), and a night temperature of 65-75°F (18-24°C), will be required. This can be achieved in summer by putting pots upon a greenhouse staging, or upon a high shelf in a sunny closed porch or conservatory, or on a sunny windowsill, and in winter by placing pots over central heating, or upon greenhouse heaters.
Although good light is needed as soon as the seeds are up, both the seeds and the seedlings must be fully and effectively protected from the burning direct rays of the sun. This is best done by interposing a piece of thick white cotton sheeting, which gives diffused sunlight with warmth, stimulating robust and healthy growth. If the light is too strong the seedlings turn red or purple and growth almost stops. If it is too weak they go pale green, elongated and watery, and eventually stop growing and die. This shading will be needed for at least the first year, in most cases.
Germination should occur within 2-6 weeks, or less, but very occasionally takes much longer. Do not be disappointed if not all the seeds come up. A germination of 50% should be reckoned as very satisfactory; it is often much less in the case of wild-collected seed!
When seedlings show visible spine development they should be given somewhat drier conditions, otherwise the developing roots will suffocate as they penetrate the soil, and will rot away. But they should never be allowed to go completely bone-dry, nor should the temperature be allowed to fall below 59°F (15°C), for at least the first year.
Cactus seedlings vary greatly in size and speed of growth. Many of the choicer varieties take a full year to get big enough and hardy enough to handle easily, and to develop a proper root. Good results are obtained by postponing the operation of potting up until this stage is reached. Quicker growth can indeed be obtained by pricking out into damp compost in shallow communal trays or seed pans at an earlier stage, but we do not recommend this delicate and hazardous operation to the beginner because unless the conditions are perfect the tiny seedlings will either dry up or decay. Instead we recommend to sow the seed sufficiently thinly (say ¼" or more from seed to seed, depending on size of seed) to permit at least a year's wait.
When pricking out or potting up it is essential that whatever compost is used should have a permanently gritty or lumpy structure, ensuring continuing good aeration and oxygenation of the roots. Composts which are powdery and deficient in air spaces, and composts which lose their air spaces by compactification over a period of time, are fatal to many of the choicer cacti and greatly suppress the growth and flowering capability of even the most resistant and vigorous sorts.
Once spines start to appear the little plants will benefit greatly by frequent application of a complete plant food in liquid form, used at a weak strength in place of ordinary water. A mildly acidic low nitrogen formulation is needed, and 'Phostrogen' fulfills both these important requirements. Mature plants should be fed in the same way.
With your seeds you are embarking upon what can be, with patience, a most fascinating and rewarding experiment. More information can be obtained from books in your local library. You say eventually decide to join a society devoted to the cultivation and study of these remarkable and beautiful plants. The British Cactus & Succulent Society has many Branches throughout the North-West, and the whole country. Meetings usually take place once a month, and you will be most welcome, to come as a visitor. For information on the Liverpool Branch and other local Branches please use links below - for link to other sites including the BCSS site goto the links page.