One of the classic gardening errors when growing cauliflowers and cabbages lies in letting the soil get too warm for them. They really prefer cooler temperatures, which isn’t normally hard in the UK, but even just a week or two of hotter days can cause the plants stress.
What happens when the plants get too hot?
With too much heat, the plants tend not to form healthy hearts – instead of producing a firm, round cabbage head the cabbages will remain loose leaved, and instead of producing a tight cauliflower, the cauliflower plant may produce broccoli-like sprouting florets instead, if at all.
How hot is too hot?
Once temperatures start to creep above 18c or 19c and into the 20s for longer than a few days, cabbages and cauliflowers will start to notice.
How can I keep them cool?
There are several ways you can help, and you may choose to do a combination of the below, especially if you live in a drier part of the country where heatwaves are a little more common.
Plant them in a slightly shaded spot
Although the plants do need some sun, they don’t need it all day long. 5 to 6 hours of full sun should be adequate, and this can be early in the morning or in the late afternoons. Having the partial shade or a few hours of more full shade will help to prevent the soil from getting too hot. As long as the soil remains reasonably cool, the plants will be able to tolerate the higher air temperatures.
Dig in plenty of compost when planting
This will just help the soil consistency and improve moisture retention a little. Be sure to firm them in really well once you have planted them though, as this will also help them establish a stronger root system and prevent the plants from rocking in the wind etc (which can be another cause of not producing cabbage heads or cauliflowers.
Mulch well around the base of plants
This is another excellent way of keeping the soil cool and if you use compost as a mulch then it will help to provide nutrients too so it’s a double win. All you need to do is spread a thick layer of compost (or other organic matter, but we really favour compost) in between the plants, particularly around the base of the stem.
Water thoroughly in the mornings
During dry spells, make sure the plants get plenty of water early in the morning that can be soaked down into the soil. The moisture will help to keep the soil temperature down a little as the day warms up.