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Good Growing

Starting a Garden: Season Extension

Cold frame with vegetables growing inside in the fall.

Are you waiting on the edge of your seat, ready for that frost-free date to pass so you can safely plant your tomatoes in the garden? If you know any vegetable farmers, they already have tomatoes in the ground. But you can’t fit a high tunnel in your backyard. Maybe the front yard? Nah, the neighbors won’t like that one bit. There are strategies to get you out in the garden sooner by extending the season. Let’s cover some early- and late-season strategies for the home gardener which don’t involve 100-foot long high tunnels.

What is Season Extension?

Season extension is exactly what it means. Now for most backyard home gardeners, there are three ways to take this.

  1. Extending the warm season crops into the shoulder months
  2. Extending the cool season crops into the winter months
  3. Or both!

Okay, not every home garden will have space for doing both, because a tomato takes up space that could otherwise be occupied by a block of lettuce, and vice versa.

While many will use season extension to give their tomatoes a head start, I prefer to use my season extension for cool-season crops. These are the forgotten vegetables, like spinach, lettuce, carrots, kale, cabbage, and many more! Everyone grows a tomato, but few gardeners grow delicious cool-season crops. Fall gardening is my favorite. As the weather cools the plants use sugars to avoid freezing, leading to much sweeter produce. Add in the cooler weather and fewer pests make it a joy to be in the garden.

We are not talking about greenhouses (though I do wish I had one). Greenhouses typically use active heating. Meaning there is some type of heat source in the structure. What we will talk about is passive heating by trapping sunlight in various types of structures, such as cloches, cold frames, and low tunnels.


Cloches are simply plastic jugs or bottles with the bottom cut off and placed over the top of plants. These are inexpensive ways to protect plants. There are all types of variations on cloches, from gardeners using an old milk jug, to cloches with waterfilled sides that hold a lot more heat. Vent on warm sunny days by removing the cap. However, since these are so small it can only hold so much thermal mass. Meaning it will cool far more quickly than a larger structure. This would not be a reliable way to keep crops alive throughout a typical Illinois winter.

Cold Frames

A cold frame is essentially a bottomless box, with a clear covering. Ideally, cold frame tops are angled and oriented to the southern sky for better interception of sunlight. Plants can grow directly in the soil inside the cold frame, or plants can be grown in flats or pots.

Cold frame construction - Often these are made of wood 1-inch thick; two-inch is more costly but provides better insulative value. I have seen these made of brick, concrete block, straw bales, or sunken below grade. Most cold frames have a repurposed window as the clear covering. You can also use plastic greenhouse film and clear polycarbonate panels. If you love the details, the slope of the covering should be 1 inch per foot from back to front to get the best angle for the sun’s rays.

Vent cold frames on warm, sunny days by propping the covering. There are some simple automatic vent openers are available that do not require electricity. These have a metal cylinder containing compressed wax that expands when heated. As the wax expands, it pushes a piston that opens the vent. As the temperature cools the wax contracts and the spring closes the vent.

Low Tunnels

Low tunnels offer a very flexible low-cost way to provide a protected growing environment to crops. The frame of the low tunnel can be made from a variety of materials. But are usually 10 to an eight-foot-long stick of metal conduit, PVC, or high gauge wire, which are bent into 3, 4, or 6-foot half-circle frames, called hoops.

A note on using PVC pipe – PVC reacts with the greenhouse polyethylene film and quickly degrades where these two materials come in contact. Several poly suppliers will not guarantee their product if PVC is used for the hoops.

The hoops are spaced at a maximum of six feet apart. Space hoops closer if you are in a high wind or area that receives lots of snow. You can secure the hoops to a raised bed frame with pipe straps/clamps. For in-ground gardens, you can push the ends of the hoop into the ground. I also install a rope along the center of the hoops to act as a spine. Stakes at either end of the low tunnel secure a rope looped around the middle of each hoop and pulled tightly. This helps with resisting wind and snow loads.

There are two types of material you can use a low tunnel covering and I like to use both. Spun row cover fabric and greenhouse polyethylene.

Row cover fabric is a plastic spun fiber material. It is porous which means it will let air and some amounts of water through, in addition to light. Row cover is what I first install earlier in the fall as temperatures begin to cool. It will provide adequate protection as nights begin to dip towards freezing.

It is also handy to use in the spring as a protective cover for plants as pest insect populations begin to grow.

Some growers and gardeners will use row cover all season to keep pests off crops that don’t require pollination, such as salad greens, turnips, broccoli, potatoes, and many others. For those crops that do require pollination, the row cover can remain on until the plant begins to flower. This can provide protection early in the season from pests like squash bugs or squash vine borer.

Greenhouse polyethylene - As temperatures continue to drop in the fall, time will come to put on added winter protection in the form of greenhouse polyethylene (poly). This plastic film will only allow light through. Which for the most part will become trapped underneath as heat energy. Ten-foot wide rolls of poly should accommodate most low tunnel hoops. The taller the hoops, the more poly it takes to span the width of the bed.

Bed lengths vary in most gardens. Simply cut the poly to fit your bed length. Remember to account for several extra feet on either end due to the height of the hoops and to secure the plastic with block or sandbags.

Even though I was harsh on high tunnels at the beginning, a lot of home gardeners, especially those in the northern US, are putting in small high tunnels in their yards to extend their growing season. 

There is so much more to consider when it comes to season extension. If you want to learn more check out my recorded season extension webinar at

Good Growing Tip of the Week: If you are using a plastic covering on your garden, make sure it is treated to withstand outdoor conditions. Your typical plastic drop cloth will quickly degrade when exposed to UV light and other abrasive environmental conditions.

More resources on season extension:

Constructing a High Tunnel

This Rutgers University website includes a 61-image demonstration of high tunnel construction, with detailed information on site selection, design, cost, and crop management.

Constructing a Low-Cost High Tunnel

Specs, diagrams, and photos of high tunnel construction, by Utah State University Cooperative Extension.

Design and Construction of a High Tunnel in West Virginia

Detailed information on hoop house or high tunnel construction, with specific considerations for West Virginia.

High Tunnels

This manual on using high tunnels or hoop houses for season extension includes case studies, a supplier list and chapters on marketing, site selection, construction and types, and soil, pest, and environmental management.

Kansas State High Tunnel Video Series

This video series was developed by Kansas State Research and Extension and includes segments on g topics related to commercial high tunnel or hoop house production.

Comparing Four Ultra-Low-Cost Season Extension Structures on a Missouri Farm

This project compared winter production using four structures: EZ Build-n-Gro Cold Frames (Farmtek); cattle panel greenhouses; modified low tunnels; and standard small cold frames.

Growing Under High Tunnels in Illinois and the Midwest

University of Illinois

Extending the Season

Cornell Factsheet

Cold Frames, High Tunnels, and Greenhouses: Chose a growing structure best for you

University of Nebraska

Building and Using Hotbeds and Cold Frames

University of Missouri Extension


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