Have you tried to sow the seeds in the open? I came across this way of sowing cacti quite by chance. I live in a Mediterranean climate area and temperatures are above zero at all times. However, the winter can occasionally be very wet and windy, and hail storms do occur. Let me first comment about the sowing technique, my challenges and then I will ask for your advice on ways to improve this technique. I will also explain WHY I have interest in this technique.

My cacti are mostly in the display area of my garden, with the familiar clay pots and gravel mulch. Seeds from those ones occasionally fall and percolate underneath the gravel. They receive all the rainfall from fall till spring. Quartz gravel is the gravel of choice. It is slightly translucent, so it protects my cacti from hail in winter while allows some light to reach the surface in the growing season. I don't know when germination actually starts, but they seem to go through periods of dry spells between irrigation and the next one in the summer, which may last up to 1 month.

The open tray/pot technique seems to provide same type of protection which the seedlings receive in the wild: enough light, some humidity beneath rocks and pebbles, yet not enough to plague them with root rot, and protection from the elements at the same time. It is like "just enough humidity for germination while just enough ventilation". I have found that pots with a single cactus are more adequate than those used for dish garden displays. I can re-pot the seedlings during a regular re-potting or maintenance. Once re-potted, their growth rate accelerates and they are ALREADY resistant to direct sunlight. Transplant success rate is virtually 100%, and I'm unaware of parallel performance among seedlings coming straight out of the sweat box.

My challenges

Not enough space, having difficulty to monitor the seedlings on a daily basis, stunted growth at the seedling phase while seedlings are not yet ready for transplant. The time needed for the seeds to reach the "out of the box" phase is long enough for anything to go wrong, be it a fungal infection, being distracted by more urgent issues, someone/something plays with the boxes, illness, you name it. It has been, for most of the time, a side issue. The open tray technique may slow things down if you are a commercial grower, but I find that it solves my problems and things start going faster afterwards.

Improvements? Your comments are welcome...