Today the soil in London is 20.6C at eight in the morning and 33.3C by mid-afternoon. In the South West it is 21.1C and 32.3C. Most of England has had no rain at all for a week. Meanwhile the soil in Northern Scotland is 13.8C and they have had 27mm.

That is the whole problem with heatwave advice: it depends entirely on where you are standing, and almost nobody publishes the number that decides it. Check the soil temperature where you are before you do anything else, and it will tell you more than any amount of general advice.

Here is what those numbers actually mean for the next week.

Why are my seeds not germinating in this heat?

Because the soil is too hot, and most people never think to check.

Seeds germinate within a temperature range, and there is a ceiling as well as a floor. Everyone knows the floor - you do not sow French beans into cold ground in March. Almost nobody knows the ceiling, and the ceiling is what is stopping your seed right now.

Lettuce is the clearest case. Above roughly 25C at seed depth, lettuce seed goes into thermal dormancy: it will not germinate, and it will not simply wait politely either - it can shut down for the season and rot (RHS lettuce guide). This afternoon the soil at 6cm in London, the Midlands, the North West and the South West is above 32C. Sow lettuce into that and you have wasted the seed.

Rocket, coriander, spinach and pak choi behave the same way. They are cool-season crops, and hot soil either stops them germinating or makes them bolt straight to seed if they do.

What to do about it: sow in the early morning, into soil that has cooled overnight, not in the evening into soil that has spent the day baking. That is a real difference - 20.6C at 8am against 33.3C at 4pm, in the same bed, on the same day. Water the drill before you sow, cover the row with a plank or a piece of cardboard for two or three days, and lift it the moment the seedlings show.

Should I be watering every evening?

No. Watering a little every evening is the single most common mistake in a dry spell, and it is actively harmful.

A light evening watering wets the top two or three centimetres. The roots follow the water, so they stay in that top layer - exactly the layer that dries to dust by lunchtime the next day. You end up with a plant that is completely dependent on you, and one missed evening kills it.

A slow, heavy soak - a full watering can per square metre, or twenty minutes on a leaky hose - pushes water down 15 to 20cm. The roots follow it down, and down there the soil stays damp for days. A garden watered deeply twice a week survives a fortnight of this. A garden watered lightly every evening does not survive four days of neglect (RHS watering advice).

Water in the morning if you can. Evening watering leaves foliage wet overnight, which is how you get blight on tomatoes and mildew on courgettes.

What should I water first if I cannot water everything?

Water what will fail permanently, not what looks worst.

What you can leave: established roots, and brassicas that have been in the ground for months. Potatoes look dreadful in a heatwave and mostly recover. A wilting courgette leaf at two in the afternoon is not a crisis - check it again at seven, and if it has picked up, the plant is managing.

Should I mulch, and with what?

Yes, and this is the highest-value hour of work available to you this week.

A 5cm layer of mulch on damp soil does two things: it stops the surface baking, and it stops the water you have just put on evaporating straight back out. Compost, grass clippings, straw, cardboard, even a bag of shop-bought bark - the material matters far less than the fact that the soil underneath is covered (RHS mulching guide).

The rule that matters: water first, then mulch. Mulch onto dry soil and you have sealed the dryness in.

Can I sow anything at all in this weather?

Yes, but sow into moisture and sow the right things.

The crops still worth putting in now are the ones with a July window that will not sulk in warm soil: Boltardy beetroot, French beans, chard and perpetual spinach, and the fast turnips and radishes. Beans in particular actively want warm soil - they are one of the few things this weather suits (RHS French beans guide).

What to hold off on: lettuce, rocket, coriander and the oriental leaves. Not because it is too late - it is not - but because the ground is currently too hot for them. Wait a week, sow at dawn, or start them in modules somewhere shaded and plant them out once they are up.

If you are sowing anything at all, the technique matters more than usual:

What is a waste of effort in a heatwave?

What should I actually do over the next week?

Check the ground before you act. Soil temperature at seed depth is the number that decides whether the seed you are about to sow will germinate or rot, and it is not the same as the number on the weather forecast. Soil sits well above air temperature in full sun, and it swings more than 12C between dawn and mid-afternoon.

Sow at dawn. Water deeply, twice a week, not lightly every night. Mulch what you have watered. And leave the cool-season crops until the ground has come back down.

The heatwave will break. The plants you keep alive through it are the ones you will still be picking in September.