Winter Protection for Plants

If you garden in areas that get a cold winter you will be familiar with a number of winter protection schemes that are used in the garden. Humans get cold, so we think that plants also get cold and need a warm blanket, but a lot of winter protection practices are a waste of time. Many are poorly understood, so people keep doing them.

In this blog I will review what should and should not be done to protect plants in winter.

Winter Protection for Plants – Why do it?

What are the problems? Why do plants need protection? It is important to understand the answers to these questions so that you can provide the protection that they really need. There are three main problems to solve.

Low Temperatures

Every plant has a minimum temperature above which they can survive on their own. Once temperatures go below this limit, the plant dies. For many tropical plants even a bit of frost will kill them. On the opposite extreme some plants, like many alpines, can take very cold temperatures and survive.

Plants are given hardiness ratings so that gardeners know what their minimum survival temperatures are. You can read more about this here: Planting Zones and Hardiness Zones.

If you are growing a plant that can not take your cold temperatures, you will need to provide some  protection against the cold.

Moisture Loss

When people talk about winter protection most do not think about water as a main factor, but water loss is probably a bigger concern for most plants than cold.

Why is this a problem? Plants get their water through the roots. When the ground freezes, the roots are no longer able to get water but the stems and leaves that are above ground still lose water all winter long. If too much is lost, the plant dies.

Water loss above ground in winter is mostly due to winds. Plants that stay under ground like perennials and bulbs don’t have a problem losing too much water in winter because the wind does not get to the plants.

Deciduous woodies, the trees and shrubs that lose their leaves in fall, only have woody stems to survive the winter and these lose very little moisture in winter.

Evergreen woodies keep their leaves and these are active in winter. Their metabolism is slowed down, but the leaves and needles still lose moisture to the air.

Animals

In summer animals have lots of choices for food, and they prefer fresh green leaves just like humans. In winter there are few if any greens and so animals starting eating the less desirable parts of plants. Rabbits and mice nibble of the bark of young trees. Deer will eat evergreens, such as yews, even though they don’t like the taste.

Ice Damage

Freezing rain is a type of precipitation can cause thick layers of ice to build up on everything including tree branches. The weight of the ice can snap the branches and even whole trees.

There is little that can be done to prevent this on larger trees or shrubs. The exception are some vertical evergreens like junipers and cedars (more correctly known as Thuja occidentalis or eastern arborvitae). They have very thin branches that bend badly with even small amounts of ice on them. In spring the branches don’t return to their normal shape which results in a crummy looking plant.

These junipers can be protected from ice damage, but a better solution is not to grow them in the first place.

Protecting Non-hardy Plants

Gardeners in colder climates can’t help stretching the growing zones. We all want to grow things that are not hardy in our zone but it is best not to plant such things. They just make extra work in fall and spring.

Unless you add an electrical heat source, the only way to keep plants warm is to trap the warmth coming from the soil. Soil is warmer than the air because heat moves from the center of the earth to the surface of the soil. Any solution that does not go right to the ground to trap this warmth will not work.

Styrofoam Cones

You can cover your plants with Styrofoam cones, also called rose cones. These are quite effective at protecting your plants. Here is a study I did to measure how well they work; Styrofoam Cones – Do They Keep Roses Warm in Winter?   For these to be effective, it is important that there is a good seal between the ground and the Styrofoam cone.

Pile of Leaves

You can also build a pile of leaves to protect plants. Form a circle of chicken wire and fill it with leaves. This does a very good job at protecting tender underground perennials. I am not sure how well it works for above ground plants but it would offer some protection. The farther you get away from the soil, the less heat you will trap.

Mulch or Soil

Adding a think layer of mulch  or soil over top of the plant works wonders and is a common practice for protecting roses. The added material needs to be removed in spring.

Mulch also prevents the freeze-thaw cycle in winter. Many people think mulch is used to keep plants warm, but that is not its main function. It is actually used to keep the soil frozen especially in early spring, when you have warm spells.

Mulch should be on the garden all year long.

Solutions That Don’t Work

Not all of the advice you see will work. Wrapping plants does NOT keep them warm. To read more on this have a look at Are Wrapped Trees Warmer in Winter?

I’ve seen people suggest that wrapping plants in bubble wrap will keep them warm on a balcony – that’s not true. The only way such wrapping works is if you extend the bubble wrap right to the ground so that you trap the warmth coming from the soil. On a balcony, there is no warmth to trap.

It is true that bubble wrap acts like a small greenhouse and traps the warmth of the sun during the day, but that probably does more harm than good. It does not take much sunshine to overheat the plant and if that happens they start to come out of dormancy. A bit of cold will then kill them. Bubble wrap should be covered with a light proof material.

Preventing Moisture Loss

As stated above, perennials, bulbs and deciduous plants do not have this issue so there is no point in trying to prevent moisture loss for them. Evergreens can dry out in winter.

Wrapping Plants

Wrapping in burlap will reduce the wind, and therefore will reduce moisture loss. Provided that you select the right plant for your location, this should only be required the first year after planting. Once the roots are well established there is no need to wrap plants to prevent moisture loss. I don’t even wrap first year plantings.

A clear plastic wrap will work better than burlap in keeping the wind out, but on a sunny day, plastic warms up the plant too much. Transparent plastic should only be used if it is covered with a light tight cloth to prevent heating  or if it is kept well away from the plant so that warm air can escape.

When you do wrap the plant, make sure to cover the whole plant, not just the lower sections as in the photo below.

For more about wrapping trees have a look at Should Trees be Wrapped in Winter?

Anti-desiccant Spray

Anti-desiccant sprays consist of chemicals that coat the leaves/needles so that they lose less water. They usually create a barrier over the stomata which is a leaf pore through which the leaves breath – take in CO2 and release O2 and water.

These sprays do work, but they work by stopping the plant from properly exchanging CO2 and O2 which is not good for the plant. They solve one problem and cause another one. If you do decide to use them, use them only on first year plantings and in the middle of winter.

Keeping Animals Away

There are several ways to keep animals from eating your woody plants.

Tree Guards

A tree guard is a spiral, plastic wrap for putting around a tree trunk. It works quite well, but is only suitable for trees with a single trunk. It is useless for shrubs. The nice thing about them is that you can leave them on all season and they will automatically expand as the trunk gets thicker.

Chicken Wire

Chicken wire is my preference. It comes in various widths so that you can cover the plant high enough to keep out rabbits walking on top of the snow. It can be cut into various sizes to fit single trunks or wide shrubs. It can also stay on all season long, although it is not great looking.

Don’t make a tight wrap with it. Keep it nice and loose so that small animals can’t reach the bark by get their snouts through the holes in the wire. It is also a good idea to firmly connect it to the soil so small rodents can’t climb under the wire. I use small sticks, pushed into the soil at an angle through the holes in the wire, similar to a tent peg.

Burlap Wrap

This material is not suitable for small animals like rabbits and other rodents – they just crawl under it, or through holes in the joints. It does however keep deer from eating the shrub. For deer, the burlap wrap does need to be at least 6 ft high, and 8 ft is better.

Animal Repellent

This will be a chemical concoction that is sprayed on plants. The animals don’t like the smell or the taste and leave the plants alone.

There are commercial products and home made remedies. The commercial products do work, but they need to be sprayed frequently, especially after heavy rains. They are not foolproof and quite expensive. Home made solutions are less effective.

Preventing Ice Damage

Ice damage of upright junipers and cedars can be prevented by wrapping in burlap or using just string.

Wrapping with Burlap

It is important to wrap the whole plant, not just the lower part. To be honest such trees should not be planted in a climate that gets ice damage. Besides, sooner or later they will be too tall to wrap and wrapping just the bottom part of a tree looks silly.

It is possible that the wrapping in this picture was done to keep deer from eating the trees.

String Wrap

Another effective way to prevent damage from ice is to use just string and tie the loose branches tighter to the trunk of the tree. Tie the string onto the trunk below the lowest branches, and start winding it around the tree in a spiral, as you move up the tree. Tie the other end of the string at the top of the tree.

Wooden Structures

Some people erect wooden structures to keep snow and ice off plants. This certainly works, but looks ugly all winter. Is it really worth doing this? Why not replace the plant with something that does not need such protection?

References

1. Photo Source for blue wrapped trees; Alan Hunt
2. Photo Source for burlap wrapped evergreens; Buffalo Spree–A Magazine of Western New York