There are a lot of different ways to protect the plants that you are growing for the winter months. To do this effectively, there are a few things that you need to understand.
You should be growing plants that are hardy in your growing climate; therefore they will be tough enough to make it through the winter just fine. Greenhouses are for growing plants and while they are effective aren’t necessarily needed for storing plants over the winter months.
A greenhouse usually has a double layer of clear plastic with air blown between the two layers for insulation as well as a heat source and a built in and automated ventilation system to maintain a constant temperature conducive to good plant growth. A greenhouse is designed to make plants actively and vigorously grow during the winter months. An over wintering hoop house has a single layer of white plastic and no heat or ventilation at all. An over wintering hoop house is designed to stay cold inside.
Hardy plants must be allowed to go dormant for the winter. You cannot drag them inside and keep them warm all winter. That would the same as me keeping you awake for four months straight. Plants need to sleep during the winter, it is their resting period.
In northern climates nurseries cover plants not to keep them from freezing because they still freeze even when covered. They cover them to keep the wind off of them and to keep them moist. Plants need moisture and during the winter when you can’t be out there watering because all of your outside water is turned off for the winter, the last thing you need is the wind pounding on the potted plants sucking the moisture out of the potting soil.
When you are covering plants for the winter you should cover them with white plastic because the white plastic reflects the rays of the sun and keeps the plants under the plastic at a uniform temperature. They might be frozen as hard as rock under the plastic and that’s fine because they don’t mind, really they don’t. What plants don’t like is to thaw out on a warm day, then freeze again that night and have that cycle repeating all winter. The constant freezing and thawing is not good for plants.
Clear plastic is used on a greenhouse where you want the sun to warm up the greenhouse. However, without a heating system in the greenhouse as soon as the sun goes down things can freeze. That’s why you should never use clear plastic for over wintering plants. It heats them up during the day, tricking them into thinking it us spring, then it allows them to freeze at night, causing them serious damage.
White plastic is hard to find because most hardware stores and home improvement stores don’t carry it. You have to get it from a nursery supply house or you can use clear plastic then after the plants are covered slosh white latex paint over the clear plastic. This isn’t the perfect solution, but it does work.
I always use Potentilla as an example of a plant that seems very frail and tender, yet come winter they are amazingly tough, even as tiny, spindly rooted cuttings. During the winter months I see them in propagation boxes, with no leaves and skinny as skinny can be. I wonder how they can possible survive temperatures well below freezing, often down in the single digits.
Then come spring as I am walking by the propagation area I can’t help but notice them, covered with little tiny green buds just busting at the seams to open. I always wonder how something so frail can be so tough. I don’t know why, all I know is that all of my rooted cuttings respond the same way. Come winter they just hunker down and wait it out. They know they have a job to do and they do it.
Our job for winter is to not interfere with what the plants already know how to do because they know how to survive the winter. The trouble starts when we interfere. We interfere by putting plants in pots. It’s not natural and it’s not normal for them. Our only job is to compensate for the situation that we created with some simple steps.
In most cases you don’t have to protect plants in the ground. Most of the time, plants in the ground are safe because it’s a natural environment for them. They are wired, to survive the winter simply by being planted in the ground where they are supposed to be.
So why are plants in pots so different? Plants in pots are not in the ground, they are above ground. They are potted in a growing medium that is designed to drain well and drain fast. That’s perfect for the growing season, but not so great for the over wintering season. When we put plants in pots we do things to them that is not natural. We put them in a well drained potting mix, and then we set them on top of the ground and let the wind pound on the thin plastic containers. We let the sun warm them during the day which dehydrates them because the only thing between their precious roots and the harsh elements is a thin piece of plastic. We actually prevent their roots from reaching out into the soil where they can pick up the moisture they need. In other words we rob them of the natural warmth of the ground.
Do not underestimate the warmth of the earth. Think about an igloo, made of snow, yet it is much warmer than the outside air. Ground heat is precious to a plant grower, use it to your advantage and never do anything to separate your plants from the precious earth.
Low hoop houses are close to the ground and we already discussed the advantages of ground heat. They have much less dead air space between the top of the plants and the inside top of the hoop house. That means higher humidity and a much lower need for you to water over the winter. However, it certainly won’t hurt for you to poke a hole in the plastic and spray the garden hose around inside of your hoop house once a month or so. If things are frozen and you can’t water, then you can’t and chances are your plants will be fine. Just make sure they are plenty moist and or snow covered when you cover them trapping in all of that moisture.
Snow won’t hurt your plants and most cases it’s good for them. Snow is plenty moist and is a great insulator. When the ground is frozen and a heavy layer of snow falls, the ground will actually thaw under the snow because the snow keeps the ground protected from the harsh cold air and the natural ground heat from below thaws the frozen soil. That’s an important lesson to grasp because you can use that information to your advantage as a grower.
I mentioned covering your plants when they are snow covered but often times it will snow before you have a hard freeze. Plants need that hard freeze where the temperatures dip below freezing for several hours in order to trigger them into dormancy. You can have snow fall, but still not enough of a hard freeze to trigger dormancy.
Big growers use large and high hoop houses even though lower houses are better. They do this because they are big growers and they do things in big ways. They like really wide hoop houses that hold lots of plants plus they like to maximize every square inch of space they use. Narrower hoop houses mean more driving isles between the hoop houses and that’s a lot of wasted space. So they make the houses really wide, up to 40′ wide. That means in order for the structure to hold up to a snow load it has to be much higher in the center in order to have the support that it needs.
Big growers leave their hoop houses up year round; they just cover them with white plastic for the winter and uncover them in the spring. Big growers like high wide hoop houses because they can walk through them and even drive tractors through them. Keep in mind that the structures are up year round and that they work inside of these hoop houses daily.
A high hoop house unless it is professionally designed is more likely to collapse from a snow load. High hoop houses have a lot of dead air space between the plants and inside at the top of the hoop house. That makes it really difficult to maintain a high enough level of humidity. Because of that, big growers have to water inside of those big houses about every three weeks.