Materials
Materials for extending the season vary depending on the setup you're building or using. The simplest coverup requires a mere plastic milk jug. Each of the descriptions below identifies materials needed for specific setups. You can also purchase some season extenders through the Gardening with Kids store. See the Resources section for details.

Creating Plant Coverups

The Basics
Most plant cover ups — cold frames, hoop houses, row covers, hot caps, and so on — whether commercial or homemade, work on the same general principle: they trap the sun's heat, which encourages plant growth. Some cover individual plants and others cover a row or bed of plants. Depending on your climate and gardening focus, you can use them to start or harden off seedlings for transplanting, boost plant growth (hasten fruit production!), or protect plants from cold temperatures, frost, and even insects.

Whichever types of setup your students create or use, it helps to know a few basics. Although most plant coverups are designed to trap heat, too much defeats the purpose. It is important to have some way to vent your setup to release excess heat, and to have your keen observers monitor temperatures. Even in the middle of winter, the temperature inside a cold frame can climb above 100 degrees! Ventilation also helps prevent diseases, such as damping off of seedlings.

Different materials have different abilities to insulate and therefore protect plants from cold temperatures. A plastic milk jug placed over spring seedlings may only offer a couple of extra degrees of protection, for instance, but a coldframe with insulated sides or a double layer of glass offers a much larger boost. The more airtight your covering is, the better protection it offers on cold nights, but the more it will heat up during the day.

It's also important to remember that some crops, such as cucumbers and squash, rely on bees for pollination, so when blossoms appear, you'll want to remove your protectors so bees can access the hidden nectar rewards.

Below, we describe several approaches to extending the season by covering plants. But don't feel limited. Once students understand the basic concepts, they can invent their own setups from recycled materials. Witness the creative scheme in this photo.

Protecting Single Plants

One of the oldest strategies for boosting plant growth and protecting them from cold is covering them individually with a type of mini-greenhouse. French gardeners in the 19th century used glass domes or bell jars, called cloches, the French word for bells. Today, commercial versions of these individual coverings are made of lighter weight materials and are easier to vent. These include umbrella- and lantern-shaped cloches (see photo), plastic "bells," and waxed paper "hot caps."

Rather than buying such devices, why not challenge your students to invent their own from recycled materials? Plastic gallon milk jugs with bottoms cut out can get garden seedlings off to a good start. (Removable lids or holes in the sides make great vents.) Tomato cages covered with plastic are an elegant solution for protecting larger plants. What else can your designers come up with? As plants grow larger, be alert for heat-damage that can occur as leaves touch the sides.

Covering Garden Beds

When you want to extend the season, or urge it onward, and have wide rows or beds, you have a few general options, which are described below.

Using Fabric Covers
You can lay fabric row covers made from spun bond polyester, which is like fabric interfacing, right on your plants, securing the ends with rocks or boards, or burying them in soil. These covers keep insects out and trap some heat in while enabling light and water to enter and excess heat to escape. You can leave them on in spring and summer, depending on your plants' heat and pollination needs. Heavier weight covers made from polypropylene offer good frost protection and warmth in the fall.

Don't be limited by the types of bed coverups described here. Challenge students to invent their own means of providing shelter and extra warmth for garden beds.

Shade for Hot Weather
Hot weather can stress and stunt plants and cause cool-weather crops (like lettuce) to go to seed. Shade netting or wood lathe attached to a tunnel or frame can provide cooling shade.

Building Cold Frames

Simply put, cold frames are mini-greenhouses that are close to the ground. They are typically used to start or harden off seedlings in containers or grow cool-weather crops, such as lettuce, directly in soil in spring, fall, and winter (depending on the region). You can also use them for rooting cuttings of perennial flowers or shrubs, storing root crops in the winter, or chilling bulbs for forcing. The Resources section has links to different plans for building cold frames, although you can readily create a simple setup without them. Here we describe basic cold frame structure, placement, and uses.

Basic Construction
A standard cold frame is 3 feet wide and 4 to 6 feet long rectangle made from hay bales (the simplest system), 1- to 2-inch lumber made from rot-resistant wood (such as cedar), concrete blocks, or lumber made of recycled plastic. The front is 10 to 12 inches high and the back is 18 inches high. (This slope is not necessary, but having a sloped lid helps catch more sunlight during cooler months.) A couple of inches usually extend below the ground level for extra insulation. It is covered by a lid made of glass, rigid plastic, plexiglas, or 3-mil plastic sheets. This top either slides off or opens up via hinges. Old window sashes and doors make great lids. In any case, you'll need to vent the cold frame by sliding the lid off, propping it up, or buying thermostatically controlled arms that open the lid when temperatures reach a specified level. Be sure to attach a thermometer so your students will know when to vent the frame.

There are certainly many variations on this theme. We would love to learn about what your students design. Here's a clever setup — a moveable hoophouse style coldframe — created by one of our staff.

Location and Use
You'll ideally want a southern exposure that is somewhat sheltered from the wind. Consider placing a cold frame up against a wall or fence, or even using a wall as the back part of the structure. If you want to grow plants directly in soil in your grow frame, dig the soil about 4 to 8 inches deep and add compost.

One of your students' challenges will be to ensure that plants do not overheat on sunny days and that temperatures inside the frame don't drop too far on cold days or at night. In general, if the temperature goes above 75 degrees F, vent it. You'll typically want to close the lid before sundown. If cold temperatures threaten, you can cover the lid with a blanket, roofing paper, a tarp, or rigid foam sheets weighted down with rocks. Another way to hold in heat is to keep plastic milk jugs of water inside against the back (the north side). If you paint them black, they'll absorb even more heat and release it at night.

If you want to use your cold frame to store root crops, such as carrots and beets, so you can harvest them, unfrozen, through the winter and early spring, dig a hole a foot deep in the bed and line it with straw. Put in the vegetables and cover them with straw, then cover the frame with a tarp.

Heating it Up
Some people who want to grow plants outdoors when outside temperatures are much colder — even below freezing — turn cold frames into "hot beds." These are simply cold frames that use another source of heat in addition to the sun. This can mean digging up to a foot of soil from inside the cold frame, adding at least 4 inches of fresh manure, and topping that with several inches of soil. As the manure rots, it generates heat. You can also use electric heating cables or other heat source instead of manure.

As you build and/or use plant coverups, students' questions should provide fertile ground for further research and investigations. See Curriculum Connections for some ideas.

Other Season-Extending Strategies

If you put mulch on your garden in the winter, pull it off early in the spring so the soil warms up and dries out more quickly. (Raised beds also dry and warm quickly, enabling you to plant early.) Use black plastic or IRT (infrared transmitting) mulch to keep beds warm. Make holes in the plastic for planting seeds or seedlings. Choose plant varieties suited to your climatic extremes (e.g., broccoli suited for spring and fall or heat-tolerant spinach). Even in the cold north frost-resistant crops such as Brussels sprouts, kale, leeks, collards, and some winter-hardy lettuce will last through hard frosts, and some years, through the entire winter. Store root crops, such as carrots, beets, parsnips, right in the ground. Cover a section of a bed with at least 6 inches of organic mulch (chopped leaves or straw). You can pull off mulch and dig them, unfrozen, through the winter. In arid areas, consider using drip irrigation or soaker hoses to get water directly to plant roots so they can keep on keepin' on!

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