With a heat wave forecast for this weekend, don’t forget to protect young vegetables from excessive heat. Depending where you are on the coast, temperatures might range as high as 26-30oC [79-86oF] over the weekend, dropping after Monday. While it doesn’t look too prolonged, these temperatures would certainly be too hot for tiny seedlings, newly sown seeds and plants that were transplanted recently—though of course, established heat loving crops, such as tomatoes and cucumbers, should enjoy it and snap out of their doldrums from the colder weather last month.

No matter how well beds are watered, at these temperatures the roots of small plants are close enough to the soil surface to fry in the midday heat. Plants large enough that their leaves shade the soil around them, are much less at risk from heat injury, however, it has been such as dry spring (despite those showers in May) that you may not realize that larger plants could be experiencing drought conditions at their roots. I was surprised to discover last week that even the soil in perennial beds, such as strawberries and asparagus, was very dry. When the soil is dry enough that plants aren’t able to extract sufficient water, they are much more likely to suffer leaf injury (sunscald), even at temperatures that don’t seem to be all that hot.

The main precautions to take immediately: shade vulnerable plants, water well and spread mulch.

If you were planning to sow more seeds this weekend, it is probably safer to wait and sow after it cools off. If you still have seedlings that haven’t been planted out, hold them in deep shade over the weekend and plant them after the heat wave. If newly sown seed haven’t come up yet, shade the bed with opaque covers such as burlap, old bedsheets or cut open compost bags, spread out white side up. Check under the covers every day and remove them the minute you see tiny shoots emerging.

Shade seedlings and newly transplanted plants with horticultural shade cloth (get a fabric that provides 30%-50% shade) or use wood latticework covers, plastic seedling trays turned upside down or anything else you can devise to throw partial shade. These let in enough light that you can leave the shading in place until the heat wave is over. You can also lay lightweight fabric, such as sheers or lace curtains or old tablecloths from the thrift shop on top of plants to shade them. These don’t let in enough light to be left in place continuously, so cover plants from mid-morning to late afternoon, then uncover to expose them to direct sun early and late in the day. If you are using floating row covers or insect netting over carrot and cabbage family beds to keep out root flies, just lay the shade material over the top of the insect cover.

Prevent temperatures in greenhouses and tunnels from getting too high by covering the outside of the structure with shade cloth. If that isn’t possible, shade plants inside the greenhouse. Open vents and doors wide, increase ventilation, use high speed fans to dump as much heat as possible; frequent misting to wet down surfaces and fill the air with moisture will also help to reduce temperatures.

Check soil moisture and, if necessary, water plants or run the irrigation system you use for summer watering. Be really vigilant about keeping seedbeds evenly moist: that is much easier for shaded beds, which don’t need nearly as much watering as beds in full sun.

Mulch as much as possible: Spread mulch around established plants and mulch plants in greenhouses, too, even in containers. For small seedlings, a soft fine mulch is best, such as grass clippings or dry leaves that have been crushed up so they can be sprinkled around tiny plants. Even a little bit of mulch will help cool the soil and slow loss of soil moisture. Remember when you are weeding that pulled weeds make an excellent mulch as long as they don’t yet have seeds.

Get this done this evening or tomorrow morning so your garden should be all set while you go out and enjoy a summery weekend!