There’s more to a growing season than just the number of days between spring and autumn frost dates. More than just the hours of sunlight and the soil affect your plants’ growth. The amount of heat your garden receives or doesn’t receive will directly affect growth rates and yields. Those plants needing more heat can be particularly difficult for gardeners in areas with cool summers.  

What heat-loving crops are we talking about?

Most of the vegetables we grow are not tolerant of frost, but when we talk about heat-loving crops, we’re looking at those plants that need hot days and warm nights to do well. Tomatoes, peppers, eggplants, sweet potatoes, okra, melons, and even sweet corn can all be problematic for northern growers with warm days but cool summer nights. 

In particular, warm-season crops like tomatoes and peppers slow biological activity and, therefore, growth below 50℉. Even if the days-to-maturity number fits within your growing period, that doesn’t mean conditions were right for growth. So what can we do about it?

Warm the soil before planting

Planting into cold soil (below 50°F) can stunt warm-season crops regardless of air temperature, and it's a common northern gardener mistake. Even if the frosts have passed, plunking those pepper transplants into cold soil can shut them down for weeks. 

Raised beds warm faster in spring than in-ground soil. Using raised beds for heat-loving crops can help them get off to a quicker start. But for traditional in-ground plantings, plastic mulch or even a temporary low tunnel constructed over the bed can raise soil temperature by a large margin.

Advantages of a temporary row cover or hoophouse

You might not have the space or resources for a full-on greenhouse in your yard, but you can mimic the conditions it creates for a few peppers or tomatoes. A few pieces of PVC pipe or a simple DIY wood frame and a sheet of clear greenhouse plastic make an easy hoophouse over a raised bed, and the rise in temperatures can be significant. These structures are great for warming soil before planting heat-loving crops, and for creating a warm environment on cooler spring and early summer days. 

Shop for greenhouse film at online vendors who specialize in greenhouses, and you’ll find reasonable prices. A 6-mil piece of greenhouse film 10x25 feet runs less than $30 and is large enough to make a covering for two 4x8-foot raised beds. The same idea works for in-ground beds as well. 

Anchor one side of the plastic film to the ground or bed edge, but leave space to roll the other side up or open the ends for ventilation on warmer days. Use these temporary hoophouses to get a jump on the hot weather, but then remove them when your summer temps arrive, and you’ll add weeks of heat for your plants.

Simple row covers held up off the plants by wire or other supports can also improve air and soil temperatures early in the season, getting plants off to a faster start. They’re cheaper than a hoophouse, and as a bonus, the supports used to support the thermal cloth in spring can also support bird or insect netting in summer.

Caution: Without ventilation, temperatures can get hot enough to cook plants on a warm sunny day. Roll up the sides or open the ends in the morning, then close them again in the early evening. Even a 60℉ sunny day can be enough to reach lethal temperatures in an unventilated plastic hoophouse or low tunnel.

Black plastic mulch

Black plastic mulch has been around for years and can significantly boost soil temperatures when used correctly. When applied over a smooth soil surface (not rough and blocky), soil temperature can increase by 3-5℉. Warmer soil temperatures have been shown in university trials to increase active root and shoot growth and allow earlier planting. Many trials have also found earlier and heavier yields. Used throughout the season, black plastic mulch can add weeks to your warm season.

If you decide to try black plastic mulch, look for a high-quality product you can use for more than one season. Don’t use weed barrier fabric; that’s not the same product. Look for one labeled "black plastic mulch."

Prioritize afternoon sunshine

We know our vegetables like full sun, aka more than 6 hours per day. But for heat-loving plants, choosing a location that receives afternoon sunshine can make a difference. Our hottest temperatures are in the early to mid-afternoon. If you have a choice of morning or afternoon sun, choose the afternoon location for heat-loving plants like peppers and melons, and leave the morning sun spots for lettuces, spinach, and brassicas. Prioritizing the hottest, most powerful sun for plants that love it can help overcome some of the cool-weather problems.

Take advantage of heat reflected from surfaces

Ever notice it’s hotter when standing on the pavement? Paved driveways and south- or west-facing walls can reflect heat, creating warmer conditions for plants near them. If your eggplants never quite mature, try planting them in one of these warmer spots in your yard. 

Planting near dark pavement can take advantage of heat radiated after the sun goes down. If planting near a south- or west-facing wall, stay within 18 inches of the wall for best results. 

Start the season early (indoors)

Starting seeds indoors is standard practice for many growers (or buying started plants at the garden center). We start seeds indoors for a couple of reasons: to get an early start on the season (for short-season gardeners) or because the plants are difficult to start in the garden. Tomatoes, for example, can be direct-seeded into garden beds, but we typically start them indoors and transplant them to give them more time to grow and yield. 

Transplanting seedlings that are already well-established means those plants are large and able to take advantage of the warming weather. They don't have to spend weeks developing cotyledons and their first true leaves before putting on real growth. All those weeks of warm weather can be spent growing from a nicely sized transplant into maturity. For cool summer gardeners, those plants are ready to use whatever warm weather you get as soon as a few days after transplanting.

Try short-maturity varieties

Sometimes, the problem isn’t that the summer doesn’t get hot, but merely that the length of the hot weather isn’t quite long enough. Whether your climate is influenced by a northern location, proximity to a large body of water, or higher elevation, cool summer temperatures can be a problem. Growing varieties that are faster to mature can be one way of working around the weather. 

Crops mature through the accumulation of time that is warm enough to support their growing needs, a factor called growing degree days (GDD). It’s the total amount of time above a certain temperature required for a crop to mature. A 105-day sweet corn needs more growing degree days than a 70-day variety. If your location is cooler, the accumulation of growing degree days is slower, so choosing an earlier-maturing variety can help. For example, try ‘Early Girl’ tomatoes (57 days) instead of ‘Red Brandywine’ (90 days). 

No one practice will fit every gardening space. Experiment and see what gives you the best results. Whether creating temporary hoophouses, planting short-maturity varieties, or warming the soil with black plastic mulch, find what works best for your location. It might be a mix of several methods.