How season extenders can help you get more out of the growing season

Karen Ramsey works with the flowering annuals in one of her greenhouses at Ledgewood Gardens in Orrington, which she opened 31 years ago. | Photo by Linda Coan O’Kresik

Especially if you live in a cold area, the natural limitations of the growing season can be frustrating. Not only does the short growing season often restrict what you can grow, but it also limits the time that you have access to fresh produce out of your garden. Luckily, there are tools and strategies for you to add more time to your growing season, known as season extenders.

“I would say that a season extender is any tool or strategy that helps lengthen the amount of time you can grow any type of plant in the region that you’re in,” Kate Garland, horticultural specialist at the University of Maine Cooperative Extension. “It includes both strategies and methods and equipment.”

Garland explained that most of the time, season extenders will either warm the soil or warm the air around the soil in order to create a more hospitable microclimate for the plants to grow, even in cold conditions in the late spring, early fall and sometimes even winter.

“The goal is to warm up the soil so seeds do not rot when they are planted and increase the rate of productivity so germination will happen faster,” Garland said. “When you talk about season extension it’s good to think about the beginning and end of the spectrum.”

Though there are many tools that are considered season extenders, Garland said that strategies like starting seeds indoors can be considered season extenders as well.

“When we start seedlings indoors, that’s a form of season extension,” Garland said. “You’re getting a jump on the season instead of directly sowing seeds inside.”

Season extenders are generally more commonly used in colder areas, but they can even be useful in areas with longer growing seasons, especially at the beginning of the season to protect against unsuspected weather and the end of the season in order to allow for another cycle of crops.

“It’s [to protect against] that late freeze, that unpredictable weather after you’ve gotten everything already planted,” Wendy Wilber, statewide Florida master gardener coordinator at the University of Florida. “We also do a second spring garden, where we plant tomatoes and peppers and eggplants in August and then that means we’re racing the clock to get them to produce before the first freeze in December.”

Types of season extenders

When many gardeners think of season extenders, the first thing they think of is a greenhouse, which allows you to grow all year round. There are other methods, however, that are less capital and energy intensive.

“There are so many more low-cost strategies that can work very, very well,” Garland said. “I tend to recommend going with lower cost approaches first.”

A hoop house, or high tunnel, is a simple greenhouse-like structure made from a frame and a single layer of greenhouse plastic. The sides are fastened to a movable bar that can be manually rolled up in the morning and down in the evening for temperature control. Low tunnels, or mini-hoop houses, which are also essentially row-sized greenhouses that are just tall enough to cover the plants. They are a less energy-intensive option that is great for small-scale growers, though it requires some maintenance and it is important to monitor them for pests.

“If folks are committed to ventilating, plastic low tunnels that can be a great strategy,” Garland said. “You have to open up in the morning and close it up at night, which can be a lot more babysitting than people want.”

Cold frames, which are effectively mini-greenhouse boxes with a clear lid that warms soil and provides light while protecting plants from the elements. Unlike greenhouses, they do not require electricity to heat, and they are especially effective for growing cold-hardy greens during the winter.

“It provides a lot of protection from the wind, but you can give it access to more light in the colder temperatures this way. I have a friend that used a cold frame all winter long,” Garland added. “Cold frames can be used at the beginning of the season, too. You can harden plants off in the cold frame.”

Cloches work similarly, but each dome-shaped, ventilated cover is usually designed to fit an individual plant. Plus, they are easy to DIY.

“You can make cloches with milk jugs and remove the cap to ventilate during the day,” Garland said. “You can get mold or pathogen issues, so make sure it doesn’t get too damp in there. You want to keep an eye on things.”

Plastic mulch is a useful season extender to warm the soil where you have planted. Mulches also help retain moisture in the soil. Black is especially popular to trap heat, though there are other colored mulches available as well.

“Black plastic [mulch] is a nice tool,” Garland said. “We use black plastic mulch as a weed barrier and a season extension tool. The disadvantage is that it’s made of plastic.”

Garland is a fan of a simple row cover, or a frost blanket, which is a spun-bonded fabric-like material that you can buy by the roll at hardware stores.

“You plant and then put [the row cover] over it. You basically make a mini greenhouse, but with cloth,” she explained. “Put it over the crop to increase the overnight temperatures. It can be used as an insect barrier throughout the whole season.”

Wilber also uses frost cloth in Florida to protect crops at the beginning and end of the season.

“We always use frost cloth to get us through, but making sure you’ve chosen your frost cloth wisely for the thickness, provide correct insulation, but not so thick that it weighs the plants down,” Wilber said.

Row covers can also be made of perforated plastic, but Garland recommended fabric and not just because it helps reduce your plastic use in the garden.

“You can also keep it on during the day as opposed to plastic approaches,” Garland said. “Can easily fry your plants with plastic, strong temperature fluctuations that can be really damaging to plants if you don’t ventilate it. Clear plastic encourages a lot of weed growth.”

In general, Garland said to look out for expensive options because they can easily be replaced with DIY options.

“There are a lot of gadgets out there. I’m shocked to see the expense of some of the pieces,” Garland said. “You don’t have to spend a lot of money to get the same effect by using row cover or well-monitored plastic.”

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