Considering that spring is just around the corner, it may seem strange to be talking about ways to extend the growing season of your garden. However, all of the solutions presented here can be used not only to extend the growing season past the first frost of winter, but also to give seedlings an early start (protecting them against a possible last frost in the the spring) and harden off young seedlings that you have started indoors.
Plus, even if you don't have anything planted yet, now is a good time to start thinking about what kind of structure you'd like to create, and how you will use it. You've got plenty of time to gather your materials - you don't want to be running to Home Depot in the fall because the first sudden frost is predicted for that night!
The easiest and most inexpensive method is a row cover - basically a blanket for your plants. They are draped right over the plants, with no supports (in some cases), and provide a heat increase as compared to the ambient temperature.
There are several different "weights" of fabric that can be used as row covers. The lightest fabric allows about 85% of the sunlight to pass through, but only provides a few degrees of frost protection. Heavier covers can provide up to 10 degrees of frost protection, but may only allow 50% of the sunlight through. These covers are best used in later fall and winter when your plants are not needing as much sunlight. Also, you may need to use wire hoops or PVC pipe to keep the heavy cloth from damaging fragile plants.
It's also possible to use plastic as a row cover, but since they can raise the temperature by as much as 30 degrees they need to be monitored carefully so as not to damage your plants. Plastic covers are more commonly used in Low Tunnels (see below).
Row covers need to be anchored on the edges to keep the wind from shifting them. You could use rocks or soil, but this tends to tear the fragile fabric very easily. Instead, try using plastic soda bottles partially filled with water as weights, or make “sandbags” by filling plastic shopping bags partway with soil.
Lightweight row covers can also be used to prevent insect damage to your plants at any time during the growing season, but don't forget to temporarily remove it from flowering plants during the day to allow pollination.
Low tunnels are also known as "quick hoops", and are essentially a mini version of a hoop house. They are a step up from a row cover in that they provide better insulation than fabric cloth, while still allowing a good deal of light through.
Since the warm air in a low tunnel is trapped closer to the soil, the soil itself acts as a heat sink, absorbing solar energy during the day and releasing it slowly at night. However, because the volume of air being warmed is so much less than in a greenhouse, the temperature variations can be much more extreme; you will need to monitor them and possibly roll up the sides on sunny days to prevent overheating.
Some of the more cold-tolerant vegetable crops that a great for growing in a low tunnel are beets, broccoli, cabbage, carrots, lettuce, onions, peas and spinach.
You can build a low tunnel out of curved metal tubing, but it's easier to just bend a piece of PVC pipe. Just drive an 18" long piece of rebar 12" into the ground, and drive another one 5 feet across from it (a typical bed width). Put one end of a 10 foot length of PVC pip onto one of the pieces of rebar, and bend the other end over to the rebar on the other side of the bed. Repeat this process every 5 feet or so for the length of the bed.
An advantage to using PVC pipe instead of rolled metal hoops is that they will store flat when not needed (plus they are considerably cheaper).
Pull the plastic over the hoops and secure the edges the same way you would with a row cover. You can use binder clips to secure it to the hoops as well (and they are handy to keep it secure when you need to roll it up).
A cold frame is simply a box with a transparent lid (typically glass or rigid plastic). The lid is sloped and faces south to catch as much sunlight as possible. In areas where the days aren't as clear, they will not be as effective as a low tunnel, which is able to capture sunlight from all angles.
The walls (sides) of a cold frame are typically made from wood, but they could be made from brick of even straw bales. However, using either of these usually means making the top lid flat, rather than angled, since you would leave gaps between the bricks and the lid if it was on an angle (through which you'd lose warm air).
Sinking the cold frame 8 to 10 inches into the ground will increase heat retention significantly. However, you need to make sure the location has good drainage, since you don't want water to collect around the frame every time it rains.
On extremely cold nights, you can drape the cold frame with an old blanket or piece of carpet to provide extra insulation.
A hoop house is simply a larger version of a low tunnel. The plastic is normally stapled to wood at the bottom of each side to keep it in place, so it is not as easy to simply roll up the sides to regulate the heat on sunny days. The ends of the hoop house should have doors that can be opened and closed to allow air to flow through; this can be something as simple as two pieces of overlapping plastic (like a tent door), or an actual wood-framed door with the side plastic stapled to the frame.
Just like a low tunnel, you'll want to use clips to attach the plastic to the hoops at each end. You can use binder clips like I mentioned for the low tunnel, but for a more secure solution you can use PVC snap clamps - they hold the plastic tighter but are harder to snap off quickly, so you shouldn't use them on a area where you'll be moving the plastic regularly.
A well built hoop house (or greenhouse) can extend your growing season by the equivalent of 1 or 2 USDA Hardiness Zones. In other words, if you're in zone 6, your plants will think they are growing in zone 8.
At the high end of the scale are greenhouses - framed buildings that are normally permanent structures and are large enough to walk in. They can even be heated to grow crops year round, although it is very uncommon to do this for home greenhouses due to the cost.
Like a hoop house, a greenhouse is so good at warming up the air and soil that you have to be carefully to not allow the space to overheat. With most greenhouses it is relatively easy to add roof vents and fans to keep the air moving.
Heating The Soil
There are several ways of heating the soil with using a cover over the entire plant. Many root crops can stand up to pretty cold temperatures as long as the root itself does not freeze solid.
Clear plastic laid directly on the ground can warm the soil 4 to 6 inches down. However, you may find that because you've created an ideal germination environment you have a ton of weeds sprouting and pushing against the plastic, so just keep an eye out.
If you have a compost pile, you may have seen it steaming on a cool day. If you dig a trench in your garden, fill it with compost and then cover with topsoil, you've created a "hotbed".
Deep mulch can be applied to protect root and stem crops from freezing as winter approaches, and extend the harvesting time for these crops. You can use anything that will trap air and insulare without getting too heavy, such as straw, leaves, burlap sacks, shredded paper (or cardboard) or bark.
You can combine almost any of these techniques - for example, you could have low tunnels inside a hoop house for added protection, or create a hotbed inside your cold frame.
How do you extend the growing season where you live? What's the latest you've been able to harvest your crops?