Maintaining warm soils during a cold summer

Consider these smart ways to keeping soils warm and warm-season vegetables healthy during a colder-than-normal summer.

Photo credit: Leon Roberts, Wikimedia Commons
Photo credit: Leon Roberts, Wikimedia Commons

The weather experts are talking about summer 2014 being a slightly cooler summer than usual for Michigan. This is because of the amount of ice that formed on the Great Lakes and the depth to which it froze. Westerly winds blowing over ice or cold water could make Michigan cooler. This may be great for the air conditioner bill, but it could make some warm-season vegetables in the garden not as productive.

First in line as a warm-season diva is the tomato. With its ancestors hailing from Central America and Mexico, tomatoes have always been somewhat disappointed with Michigan’s weather. However, a cool summer season could slow plant growth and fruit production. Other warm-season vegetables that could be similarly affected are peppers, eggplant, tomatillo, okra, melons, squash, beans sweet corn, cucumbers and pumpkins.

The question becomes what can the home gardener do to keep and maintain a warmer soil? This is a challenge for smart gardeners rather than a problem.

The very first thing Michigan State University Extension recommends is to make sure these heat-loving vegetables receive eight or more hours of full, uninterrupted sun. The more direct sun, the more soil warming will take place. Making fruit requires a great deal of energy and the sun enables those warm-season vegetables to produce a bountiful crop.

Another possibility would be not using organic mulch like straw on some or all of your warm-season vegetables. Mulch is invaluable for keeping soil temperatures down because of its insulating ability. Bare soil that has been loosened will absorb the sun’s rays. This is when a technique called “dust mulching” can work. The soil is lightly tilled or hoed to a depth of 2-3 inches. The loosened soil dries out and heats up. The compacted soil below the tilled area holds in moisture. This also prevents weed seeds from germinating because there is no moisture at the top where seeds would germinate. The drawback is that you have to continue to dust mulch if you are walking on the soil or overhead watering, or if it rains.

If bare soil works for your garden, you could distribute compost on the soil surface. Because it is dark in color, it absorbs the sun’s rays even better than many soils that are various shades of tan. Adding 2 or 3 inches on the soil surface gives the garden a double treat. The soil is warmer and at the end of the season, the compost is tilled into the soil to work its magic on plant roots in coming years.

Another technique is to cover the bare soil with black or clear plastic. This will allow heat to penetrate and the plastic will hold it in more successfully than just bare soil. It is important to anchor the plastic so windy days do not cause your plastic to run away from home. Rocks, bricks or scrap pieces of lumber could be used. You could bury the edges in the soil, but as the plastic traps heat, it also blocks rain and overhead irrigation. Watering with a hose placed close to the base of the plant will get the water under the plastic or by laying a soaker hose under the plastic. Moisture can be directed to the root zone by only hooking up one end with the other closed off.

If using plastic, buy the thickest plastic that you can find. Thin plastic does not hold up well when walked on and picking dozens of little pieces of torn plastic embedded in the soil could cause most gardeners to invent new and colorful expressions.

Watch the weather and temperatures as the growing season continues. Be ready to employ any of these heat-saver techniques if this will help your garden.

For more information on a wide variety of Smart Gardening topics, visit the Gardening in Michigan website at or contact MSU’s toll-free garden hotline at 1-888-678-3464.

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