Can I sow seeds during hot weather?
Some seed refuses to germinate above a certain soil temperature rather than merely germinating slowly. Which crops are affected, and the techniques that get a summer sowing to come up.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 4 min
Short answer
Lettuce is the notorious one — above roughly twenty-five degrees the seed can enter thermodormancy and simply will not germinate. Sow in modules somewhere cool and shaded, water the drill before sowing rather than after, cover with light-coloured material rather than dark soil, and sow in the evening.
People assume a failed summer sowing means old seed or birds. Often the seed is fine and has actively decided not to germinate — a genuine dormancy mechanism that stops a plant emerging into conditions it cannot survive. Working with that, rather than sowing more seed harder, is the whole technique.
Step by step
- Know which crops are heat-sensitive at germination.Lettuce is the strongest example and can shut down completely in warm soil. Spinach, some brassicas and pansies are also affected. Beans, courgettes, squash, sweetcorn and most Mediterranean crops are the opposite and want warmth. Carrots and parsnips germinate reasonably but need constant moisture, which is the hard part in summer.
- Sow into modules in a cool place.The reliable technique for summer sowing is to control the environment rather than the weather: sow into modules, keep them somewhere cool and shaded — indoors, a north-facing windowsill, a shaded cold frame — and plant out when they are ready. Soil temperature in a module in shade is far lower than in an open bed.
- Water the drill before sowing.Draw the drill, fill it with water, let it drain away, then sow into the wet drill and cover with dry soil. This puts moisture exactly where the seed is, and the dry cover on top acts as a mulch rather than crusting. It is much more effective than sowing and then watering the surface.
- Cover with something pale.Covering a drill with a light-coloured compost, vermiculite or sharp sand rather than dark garden soil keeps the seed zone measurably cooler and stops the surface capping. It also makes the row easy to see.
- Sow in the evening.The seed then has the whole cool night to take up water before facing a hot day, which is often enough to get germination started.
- Shade the row until it germinates.A board laid over the drill, a length of shade net on hoops or fleece keeps the soil surface substantially cooler and holds moisture. Check daily and remove it the moment seedlings appear, or they will draw up weak and pale.
- Keep the surface consistently damp.Germinating seed that dries out once is dead. This is the single most common cause of failure with summer-sown carrots and parsnips. A covering, plus a gentle water in the early morning, is more effective than a heavy soak once.
- Consider pre-chitting difficult seed.For lettuce in high summer, germinating the seed on damp kitchen paper in a sealed container in the fridge for a day or two, then sowing it just as the root tips emerge, sidesteps thermodormancy entirely. Fiddly, and it works.
- Choose what the season actually suits.Rather than fighting to germinate lettuce in July, sow the crops that want summer heat — French beans, chard, beetroot, rocket for autumn, spring cabbage, oriental leaves as the days shorten. There is a productive sowing for every month; it is not always the one you had planned.
Tips
- Fluid sowing, where pre-germinated seed is mixed into a gel and squeezed along the drill, is an old technique that works very well for summer carrots and parsnips.
- Sowing slightly deeper than usual in summer puts the seed into cooler, moister soil — but only slightly, and never for very small seed that lacks the reserves to reach the surface.
- A plank laid over a carrot drill until germination keeps the soil cool and damp and makes a real difference. Check under it daily.
- Old seed germinates worse in marginal conditions than fresh, so summer is the time to use new packets rather than the ends of last year.
- If a summer sowing fails, sow again three weeks later rather than immediately. Conditions usually improve and you lose less seed.
Common mistakes
- Sowing lettuce into hot soil and blaming the seed — Lettuce enters a genuine dormancy above roughly twenty-five degrees and will not germinate however much you water it. Sow in modules somewhere cool instead.
- Sowing and then watering the surface — It washes fine seed about, caps the soil into a crust, and much of it evaporates. Watering the drill before sowing puts the water where the seed will be.
- Letting a germinating drill dry out once — Seed that has taken up water and started to germinate is killed by a single drying. Consistency matters far more than volume at this stage.
- Leaving a shading board on after emergence — Seedlings under cover for even a couple of days after germinating become drawn, pale and weak. Check daily.
Questions people ask
Why will my lettuce seed not germinate in summer?
Thermodormancy. Above roughly twenty-five degrees, lettuce seed actively shuts down rather than germinating slowly — an adaptation that stops seedlings emerging into conditions that would kill them. Sow into modules in a cool shaded place, or pre-chit the seed in the fridge.
Is it too late to sow anything in high summer?
Not at all. Late summer is the right time for a great deal — spring cabbage, oriental leaves, rocket, chard, winter salads, autumn carrots and green manures. The list of what suits the month is different, not shorter.
Should I sow deeper in hot weather?
Slightly, for larger seed, because it reaches cooler and moister soil. Do not do it with fine seed such as lettuce or carrots, which does not carry enough reserve to push up from depth. Better to sow at normal depth and shade the row.