I figured that I have sent out so many heat alerts over the years that you all know what to do, but then remembered all the new subscribers that may not know how serious extreme heat can be for our food garden plants. The plants most likely to die from extreme heat are germinating seeds and small seedlings, of course, because their roots are close to the hot surface. The leaves of young plants are prone to being burned, but any kind and age of vegetable can experience heat injury if it gets hot enough, especially if the soil is allowed to get dry. In dry soil, heat injury occurs at lower temperatures than it would for well-watered plants.
We had a taste of hot weather already this week, but with the kind of temperatures forecast for much of the region starting tomorrow, it looks like we will be experiencing record-breaking heat. And that means it will be necessary to deploy some kind of shading over vulnerable plants. In addition to any young plants, this also includes mature leafy greens, lettuce, peas and cabbage family crops (broccoli, cauliflower, cabbage). While broccoli and cabbage will be less stressed and will perform better if they are shaded, shading is essential for cauliflower, which is very sensitive to heat. In fact, if you have cauliflower heads close to harvest now, you might as well pick them today because the heads rapidly come apart and become ‘ricey’ and strong flavoured in the heat. While you are at it, harvest Chinese cabbage, radishes, lettuce, spinach now before the heat causes bolting.
For shading, you can buy horticultural shade cloth (products that provide 30%-50% shade are ideal) or use lace curtain material. You can also make wooden latticework covers (cedar lath panels are available at lumber yards). These options let in enough light that you can leave the shade material in place until the heat wave is over. You can also use opaque materials, such as old bed sheets or any kind of lightweight fabric, but it would be best to cover your plants in mid-morning and uncover them in late afternoon so they have some photosynthesis time in the cooler parts of the day. This is NOT the time to use floating row cover (e.g., Remay), since it is designed to let in as much light as possible and to trap heat. Just remember that heat kills, while a period in the shade is merely inconvenient for plants….so use anything you can lay your hands on to cover plants for the short period it will be necessary to protect them.
For newly sown beds, cover these with burlap, opaque white plastic, old beach towels or bedsheets to prevent the soil from getting hot enough to kill germinating seeds. If you have some of those plastic lattice-work seedling trays, cover small seedlings with these, turned upside down.
For plants in greenhouses or tunnels, temperatures will be far too high without shading so cover the greenhouse with shade cloth. If that isn’t possible, then shade the plants inside the greenhouse using any kind of shading fabric. Increase ventilation, including using high speed fans, if possible, to dump the heat.
AND don’t forget to finish up mulching everything to prevent soil temperatures from getting too high and to conserve moisture. AND do increase irrigation as much as possible within the constraints of any local watering restrictions. For additional water, note that there is a lot of clean household water that can be collected and poured on garden plants, whether from the kitchen (veggie washing, dish rinsing, etc.) or the bathroom (collect shower water in a bucket while waiting for it to warm up enough to step in).
And stay cool, yourself!