Getting started -- For soil, i recommend a mix of 1 part high-quality potting soil and 1 part perlite. Most cactus soils should work. Fill small containers that have drainage holes with the soil, and water so it is uniformly moist, but not soggy. Place the seeds on top 1-2 inches (3-5 cm) apart (or use separate containers). Sprinkle a very thin layer of soil over them - just enough to barely cover the seeds, because the seeds need light to germinate. Water the top layer lightly.
They germinate well at 66-74° F (19-23°C). I have no information about how they will do outside that temperature range. For light, you may use a fluorescent bulb kept 5 inches (12 cm) away, or the equivalent amount of light. Avoid direct sun.
Until the seeds germinate and continuing for 3 weeks, ensure that the surface soil does not dry out. If you enclose the pots in a plastic dome or bag to maintain moisture, leave it open a crack for some fresh air to enter. You may still need to drip several drops of water on the seeds every day or 2.
The seeds should begin sprouting after 5-6 weeks, although allow up to 3 months for any slow ones.
After about 3 weeks, you may allow the surface soil dry out a bit, but continue keeping the lower soil moist (but again, not soggy).
Lighting -- At 1 month old, give your seedlings either filtered sun, or 2 hours of morning sun plus bright shade the rest of the day. Or continue giving them fluorescent lighting, which will keep them going for 4-5 months, after which time they should have stronger lighting.
Each month, give your seedlings about an hour more sun each day, until eventually they are in a half-day of sun, with some shade during the hottest part of the day. After about a year, they should be able to tolerate full sun, except perhaps in very hot climates.
Repotting -- When your seedlings are 1-2 inches (3-5 cm) tall, gently dig them up and plant them in larger pots. The first 2 weeks after repotting, shade the plants from strong sun, and give no fertilizer.
Long-term care -- Water the soil whenever it is about 2/3 dry down where the roots are. I don't recommend letting the soil get bone-dry like you might with cacti. Puyas can tolerate moist soil as long as it's well-draining.
Feed once a month during periods of active growth, using a general-purpose fertilizer at 1/8 the recommended dose the first 3 months, 1/4 dose the next 6 months, and 1/2 dose the next year. Most potting soil contains some fertilizer, and if yours does, your seedlings shouldn't need feeding for the first 2-3 weeks.
I recommend protecting your plants from frost for at least the first winter or two. Puyas generally flower soonest if protected from prolonged, hard frosts.
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