Gardening: Tricks for warming up the soil

Recently, I checked the soil temperature in my raised vegetable garden beds. The results were not encouraging. The temperature averaged a little short of 40 degrees; a level that is too low to sprout peas, carrots or lettuce, let alone make a tomato or pepper happy. So how do you warm up this cold soil we’ve been gifted with this spring?

Normally for cool season crops, the soil temperature should be in the high 40s while warm season crops need closer to 50 degrees to get a good start. At this point in the season, we need to warm up the soil by about 10 degrees to give the plants a chance. The warmer weather we are starting to see should be a big help in this process, but it can take a couple of weeks to warm up the soil significantly.

Raised beds and large pots will warm up faster than in-ground beds. This is because the box sides are exposed to the warmer air.

Another option is to cover beds with clear plastic that then absorbs the sun’s heat and transfers it to the soil. Clear plastic absorbs more heat than black plastic. The soil needs to be level and the plastic laid directly on it and the edges buried with soil to prevent them blowing off. Another option is a solarizing plastic mulch film that is found online that is efficient at absorbing infrared heat from the sun. I’ve used this to warm up a box by 10 degrees in two weeks so I could plant sweet potatoes. In both cases, the plastic can be left on the soil and holes made in it to plant through it. It’s best to install drip tape under the plastic for watering as the plastic is not porous.

Another trick is to rake soil into 6- to 8-inch-high mounds or raised rows to expose it to the warmer air. This is often used for planting melons, cucumbers and squash to give them some extra warmth. The plastic mulch can be added to the mounds for even more heat gathering.

After planting, floating row cover can be laid loosely over seeds and plants to hold in the heat. The material is a spun polyester fabric that lets in water, light and air through while holding in heat. Be sure to anchor the edges down to keep it in place. An even more elaborate but effective system is to create a low greenhouse over your plantings using row cover and ten-foot lengths of white PVC pipe.

The pipe is bent over the beds and anchored on pieces of rebar stuck in the soil and then covered with the row cover. The fabric is gathered at the ends and tied to stakes. I leave mine on until late June and find that tomato plants under this greenhouse will be twice as big as those left in open beds.