Autumn sowings not coming up? Your soil is too warm
You sow lamb's lettuce, spinach and salad leaves in late August, and nothing happens. Usually it isn't the seed's fault — it's a few degrees too much down in the bed.
Late August is the sweet spot for autumn and winter sowings. And yet this is exactly when rows stay stubbornly empty: you sowed, you watered, you waited, and nothing came up. The cause is nearly always the same one, and three simple steps fix it.
The problem is in the soil, not in the seed packet
Many leafy vegetables come with a built-in brake: if the soil gets too warm, the seed simply refuses to germinate rather than sprouting into a heatwave and drying out days later. Botanists call this thermodormancy, and it is really a sensible piece of self-protection.
In lamb’s lettuce that brake kicks in at around 20 degrees Celsius soil temperature, in head lettuce at roughly 25. And in late August the soil is often considerably warmer than the morning air feels, especially in a raised bed that heats up from every side.
Who strikes in the heat, and who doesn’t
The sensitive ones are lamb’s lettuce, head and leaf lettuce, spinach and winter purslane. For these it is worth the extra effort of cooling and shading.
The tough ones are radishes, rocket, Asian greens and winter radish. They germinate reliably even in warm soil, as long as you keep them evenly moist. So if your bed is sitting in full sun right now, start with that second group and push the lamb’s lettuce back a week or two. Early to mid-September still sits comfortably inside its window.
Water the drill, don’t soak the surface
The most common mistake is to sow first and then pour water over the whole bed. It evaporates off the top, caps the surface, and the cooling effect lasts barely an hour.
Do it the other way round: draw your seed drill, water it thoroughly, wait a moment for the water to sink in, and only then lay the seed in. Cover thinly with dry soil and press down gently. The moisture then sits exactly where the seed is, and down there it stays noticeably cooler for longer.
Shade it until the first green shows
Lay a plank, a wooden batten or a piece of cardboard straight onto the watered drill. Underneath, the soil stays moist and several degrees cooler than beside it, and in most cases that alone does the job.
One thing matters: from day three, check daily. The moment the first tips break through, the cover has to come off, otherwise the seedlings grow pale and leggy, reaching for light. If you know you’ll forget to check, use white shade fleece instead. It lets light through and can simply stay put.
The fridge trick for lettuce and lamb’s lettuce
If you’re in a hurry: 24 hours before sowing, put the seed between two damp kitchen towels in a sealed container and pop it in the salad drawer. At around 5 to 8 degrees it swells and gets exactly the chill phase that releases the germination brake.
Sow it damp afterwards and don’t let it dry out again in between. In practice this often buys you a good week.
One more small thing with a big effect: sow in the late afternoon rather than at midday. That gives the seed a full cool night before the next round of midday sun hits the bed.
And if it really was the seed this time
If a row stays empty even though you did everything right, check the date on the packet. Lettuce loses a lot of viability after two or three years; onion, parsnip and root parsley often after just one.
So that next time you know what came up when in your own garden, note the sowing date and the emergence date right there at the bed. In Rootivo you keep that in the garden diary, and by next August you have your own site-specific figure instead of a generic calendar entry.
Im Lexikon: Feldsalat