Instructions for growing cactus from seed.
Mix 3 parts of commercial potting compost with 1 part of grit (not sand) and 1 part of perlite.
Level down into pots or a seed tray.
Soak the compost with water.
Sprinkle the seeds evenly over the compost. Do not cover them with soil, they need light to germinate.
Place in non-direct sunlight at 70-80 degrees F (21-26C)
Keep the compost moist, but NOT wet, by spraying with water when needed.
Seeds should germinate within 2 weeks.
Watch out for damping off fungus and spray with a suitable fungicide immediately.
Keep the seedlings in indirect sunlight for about 6 months, then slowly let them have more light and less humidity. They will have a lush green colour if the light levels are right. If the epidermis turns red it means they are getting too much light, so shade with an appropriate material. Muslin or thin white cloth works well for us.
Be in no hurry to re-pot them. Wait until they are fighting for room.
Trichocereus macrogonus seedlings