Growing Vegetables in a Heatwave

Extremely hot weather seems to be more of a regular occurence these days and as I look out over my vegetable garden, I can see my plants are suffering today. Growing vegetables in a heatwave like this is certainly a challenge. This is especially true for young plants, like those that I just set out in the garden a few days ago. I've written before about how mulching can really help with locking in moisture, helping plants cope when the rains fail to come. But despite the mulch, on extremely hot days like today, young plants are often sweating out more moisture than they can draw up through their newly planted roots - especially large leaved plants like lettuce, courgettes and squash - exactly what I've just planted out. How can we help them get established?

1. DEEP ROOTS

It all starts right from the get go, with giving them the very best chances of establishing a deep root system. Deep roots reach for those deep layers of soil that hold on to moisture the longest during the heat of the summer. So during planting, there are three things to do:

a) Water the Planting Hole Deeply

This means your plant is planted above a deep reservoir of moist soil. It's roots will forage downwards as they grow and establish, instead of out and up into the shallow layers of soil, which more most prone to drying out. On my clay/loam soil, I fill my planting hole with water, let it drain and fill it again. This is enough.

b) Innoculate with Beneficial Microbes

A healthy soil microbiome creates an open, porous soil texture - allowing deep penetration of future waterings. It also means beneficial relationships can become established between your plant roots and mycorrhizal fungi, which will help your plant forage for hard-to-access water during times of drought. Having a diverse soil microbiome also gives plants access to high quality nutrition, keeping them healthy, strong and better able to withstand shocks, such as drought and extreme temperatures. Use a well rotted home made compost or a bought-in granulated product, such as Rootgrow - sprinkle at the base of the planting hole. Or use a water-in product such as Microbz Plant Boost.

c) Mulch

Lock in moisture after planting with a topping of mulch. Use a deep (4cm minimum) layer of compost, idealy home-made compost for extra soil life benefits (see above). This is ideal for all plants, but in particular small and slug-beloved plants like young seedlings, lettuces and brassicas that can't handle the slug attracting qualities of a bulkier mulch.

For larger and less palatable-to-slug plants, use hay/straw/grass clippings in a layer of around 8cm on top your compost layer (or instead of this layer if you're short on compost). The straw mulch works much better at locking in moisture and can even be made out of weedings (seed free) from the veg beds - see photo below. As I've said above, be aware that this type of mulch can harbour slugs. Not always a problem. I use it around my tomato, courgette, squash, bean and pea plants and they seem to do fine. Seedlings, young brassicas and young salad greens don't tend to do brilliantly with this type of mulch though, until they're very well established. Before that the slugs can be too much of a problem, so it's best to stick to just the compost mulch.

Weeds from the veg garden have dried out in the sun and been added to my straw mulch to add depth

2. IN EXTREME CONDITIONS - PROVIDE SHADE

It's no good providing lots of moisture, if your plants are transpiring it out faster than they can draw it up and that's often the case with new plantings that have yet to get those roots established. Help them along with some shade netting - good at any time there's a heatwave, but in particular during those early weeks of life out in the hot summer garden. I found these newly planted lettuces wilting badly in my veg garden today, but within two hours of putting up shade netting and giving them a good water, they have perked back up nicely.

My wilted lettuce immediately after putting up the netting...

...and two hours later it's looking much better

I use regular garden hoops to support my shading. Frames can also be rigged up with canes or rods cut from the garden. You basically want to create a sloped surface over which a white sheet of some kind can be fixed. The slope will face towards the sun at lunchtime, offering shade throughout the day, but in particular during those hot midday hours.

It's important to leave an opening to allow for airflow - we don't want to cook our plants in there! So while the side facing the sun is closed off, creating that all important shade, the side away from the sun is best left completely open.

This is a fine insect mesh netting, folded over to double up the thickness and provide more shade.

3. CHECK REGULARLY, WATER DEEPLY

Once all of this is set up, you should find, if you check, that your soil is remaining cool and moist underneath that top layer of mulch. Check every day during a heatwave and only water when it begins to feel dry in the top layer of that soil beneath the mulch.

When you do water, water long, water deeply and then water some more. It's really important to get that water deep down into the soil. Happily, a cool soil soaks up water much more readily than a hot soil, so you should be able to get that water down deep without too much trouble. In a hot soil it often just rolls off the bed and when you push a finger into the soil to check, it's clear that only the very surface has got wet - nothing has penetrated down. But with the set up as described above, you should have no problem with this at all.

Water either first thing in the morning or last thing at night, when it's cooler, for best results.

In an emergency, like mine today with these lettuces, get your shade up first, then water deeply. Be careful not to get too much water on the leaves as this will magnify the sun and frazzle the leaf beneath. I set up this shade netting right in the middle of the day and apart from a little bit of frazzling where wilted leaf tips were touching the ground as I watered, they're thankfully now looking pretty good.

Happy lettuces!

Good luck! I hope your veg do well in the heat this year!

If you'd like any more help with summer heat, you might like to read my article Drought-Proof Veg, with more detail about creating a moisture retentive soil using mulch and ground cover. Or if you'd like more in depth support in getting your veg garden set up, you might want to have a browse through info about my gardening 1:1 service or feel free to get in touch directly to find out more.