Always begin with fresh, dry seed.
Fill a small pot to the brim with specialty cactus soil, or mix your own. Gently press the soil down to level it off. Inorganic grit, sand, or pumice gives the soil the appropriate drainage and aeration. This type of soil composition is mandatory for cactus and succulent growth.
Just before you are ready to plant the seeds, soak them in warm water for 30 minutes or so. This loosens up the seed coat, and activates germination. Opuntia species have very tough seed coats and require a few days of soaking in warm water. After you have soaked the seeds, sprinkle them across the top of the level soil. Do not press the seed into the soil unless the seed is exceedingly large.
Gently sprinkle just enough inorganic top dressing over the top of the soil and the seed to cover completely. Try not to bury the seed in the top dressing. Water carefully and allow the pot to drain completely.
A note about watering: The use of distilled or fresh water is imperative, as to help prevent bacterial and algal growth during step 4. If your tap water tastes of chlorine, it is too chlorinated for your plants and you should consider using distilled water. The chlorine will burn the tender young roots and can cause iron chlorosis, adversely affecting or terminating growth. Careful watering can be done by either by letting the pot stand in half it's height of tepid water for a few minutes, or by gently watering from the top, taking care not to wash away the top dressing. In both cases, allow the pot to drain completely.
Cover and seal the pot with some sort of plastic container which allows light to filter through. This could be a supported plastic bag, sealed around the pot with rubber bands, or a plastic tub. The idea is to create an environment that will trap heat and moisture-just like a greenhouse. Many things will work, be ingenious and creative! However, be careful not to leave the seedling pot standing in water. Transfer the pot to an area with bright indirect light of about 70°F (21°C) if you have chosen a plastic cover which is clear and colorless. If you have chosen a container that is clear but colored, say blue or green, you will want to place it in a location with a little more light and don't increase the temperature. If you have chosen a container which is slightly fogged or cloudy, you will want to place it in a cooler location, 65°F (18°C), which gets at least 4 and no more than 8 hours of direct sun, with the remainder of the day in bright indirect light.
Remember, late morning and afternoon sun is considerably hotter than early morning or early evening sun. In general, think of the plastic container as if it were sunscreen. Clear-colorless plastic allows the most light to penetrate, clear-colored plastic allows a little less light to pass through, and fogged-colorless plastic allows even less light to infiltrate. In all cases, the inside of the plastic container will heat up.
Be careful not to fry your tender young plants! If the walls of your container lose moisture and become dry during germination, water sparingly, reseal, and return to warm well-lit location. If algal growth sets in, remove the cover and allow the seedling pot to air out a little while wiping the cover down with a no more than 1 part bleach to 20 parts water solution (5% bleach in water). Allow the cover to dry. Recover and seal the seedling pot and transfer back to a warm and well-lit location. Continue to clean the plastic container as necessary.
Although above ground the newly sprouted seedlings appear to be well on their way, below ground they have disproportionably small root systems and cannot readily absorb the nutrients they require to flourish into maturity. Therefore the seedlings should be kept in plastic container, at high humidity until they are nearly overgrown. High humidity facilitates water and nutrient absorption in the roots and through the leaves until proper roots systems establish.
After the seedlings have overgrown, protect your fingers by wearing gloves, or wrapping them with tape, and gently remove each seedling from the original nursing pot. This is much easier if the soil has remained moist.
Gently repot the seedling into a pot filled level, and to the brim, with cactus potting soil, and top dress with sand, gravel, or pumice. Water 3 or 4 days later. Do not return the cactus to the plastic humidity chamber.
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