Winterizing Shrubs, Trees and Perennials with Matt Andrus
Follow these steps from Arborjet’s Horticultural Specialist, Matt Andrus, to avoid winter injury and damage to your plants. Andrus is a member of Arborjet’s research and development team, devoted to the discovery and development of the most effective equipment, formulations and application methods.
Determine Your Plant Hardiness Zone
“First things first, with any type of plant, whether it is a shrub, tree or perennial determine the zone that you live in. Massachusetts, for example, has a plant hardiness zone number of between 5a and 7a. What that number represents is the highest and lowest temperatures a region is expected to see. We call our area here five and seven because the temperature fluctuates. Although we generally don’t get below negative twenty, it does happen, so we base the hardiness of our plants off of what extreme temperatures a region might see.
Once you determine your Plant Hardiness Zone, look into what plant varieties you have to see if they can begin to tolerate a winter. For instance, Chrysanthemums are perennial plants for our zone, yet they are sold as annuals. Annual meaning you plant it every year, as they don’t survive our winters. However Chrysanthemums are cold tolerant, it’s just that they are a shallow rooted plant that needs a little bit of help sometimes, particularly when they are grown on a commercial scale in tiny pots. It’s a good example because if you follow the procedures, one Chrysanthemum should last you forever. They are generally sold to people every year because they are not guided to insulate them for the winter so they often die but they can definitely survive if you follow these few easy steps.
Don’t Fertilize in Late Fall
You don’t want to be fertilizing after Mid-late August and the reason is that if you give the plant a nice dose of food late in the season it is going to push new vegetative growth. Vegetative meaning new branches and leaves as the plant elongates, not buds or flowers. The new tender growth won’t be able to harden or mature prior to a freeze or frost, therefore you will get a lot of damage to the plant. It may seem counterintuitive but the smaller amount of food you can use in late summer the better chance the plant has to escape damage.
In mid-August, pushing into the fall, before you put the plants to bed for the winter you have the option to divide certain species. If you want to divide you should do it before the rest of these steps.
Amend the Soil
Once divided, we want to try to amend the soil with some sort of organic matter, such as compost, manure or earthworm castings. Earthworm castings are a nutritional and insulating choice that may not be as common to locate, but acts the same as a compost. When you amend the soil, you add a layer of moisture and a layer of insulation that adds some nutrition. Please note that compost nutrients are different from fertilizing.
It is difficult to speak generally about this because there are so many different species and they grow differently, but cutting back the plants is going to make insulating them easier with less areas to become damaged by wind, freezing or frost. The more tight and compact you can keep the plant, the less chance there is of wind drying out exposed branches.
Clean Up Debris
Something a lot of people miss when they are cutting back their perennials is the removal of debris on the ground next to the crown of the plant. The crown is where the above-ground part of the plant meets the root system at the ground level. Keeping that area clear of branches, stems and leaves is going to reduce the chance of a pathogen, such as a fungi, bacteria or soil borne disease. Whatever you cut back, remove from the area and then you can either cover the crown in compost or mulch or a combination of both.
Mulching & Insulating
Provide a good 4-6 inches of mulch as an insulating barrier. Referring back to chrysanthemums as an example, if you cut the chrysanthemum back to the ground and you mulch 4-6 inches on top of it, the chances of that coming back in the spring are good. What insulating means from a plant’s perspective is when you get a shallow rooted plant, after a few mild days in the spring, if the sun is beating down on the ground it doesn’t take a whole lot of time for the sun to warm up that top few inches where then the roots will become active. If the ground starts to thaw, the water starts to move within the plant as well. Once the plant is signaled to start drawing water, it tries to unfold leaves. The problem then is any temperature below freezing will cause winter injury or frost damage, because the plants have started to pull water which will then freeze and rupture water holding cells. If you have that 4-6 inch insulating barrier, a mild, sunny day in the spring is no longer going to warm up the roots enough to draw water when there is still a danger of freezing temperatures. We want to insulate the plants to keep them from freezing as long as you can.
Many people think that the cold can hurt plants, but it’s not the actual temperature as much as it is the fluctuation that makes the plant not know whether to put out a leaf or stay dormant. If it breaks dormancy it will become injured because it can’t survive the conditions; however, it can survive those conditions if it remains dormant. This concept can be applied to many plants, shrubs, trees. Keep it cold until your frost free dates. In Massachusetts, our spring is between mid-April and mid-May, but the fall is the end of October until mid- to end-November. Mid-April to mid-May is where you have a lot of damage going on because you are going to be getting warm days and cold nights that can break dormancy. If you can keep everything insulated until the end of May, the chances of everything coming back with no damage are much higher. Similarly, in the fall things can start going dormant. Look at your frost free dates in the spring and fall and that will give you a timeline with which to prepare yourself.
Mulching & Insulating
After you amend, mulch and compost to put enough down to cover an area as wide as the branches were before you pruned them. This is called a “dripline” and should give you an idea of the size and shape of the roots. Use the diameter of the branches as a guideline. Once you have amended and mulched at 4-6 inches, you want to give the plants a nice long soak of water. The best way to do it is to leave the hose on a trickle for about an hour. Depending on how large an area you are doing, you don’t want to be wasteful with the water but the idea is to put it on a trickle to let it saturate deeply, rather than short bursts. The water and mulch work together as an insulating barrier that surrounds everything and when the soil freezes, that water freezes around the roots and keeps the soil very cold. This stops dormancy from breaking early. The mulch on top of that slows the sun down from warming that frozen water up. So with those two combined steps, the chances of breaking dormancy early are slim to none.
Shrubs & Trees
Shrubs and trees receive the same basic treatment as Perennials, with several notable differences. Conifers like white pine, spruce, and arborvitae are prone to windburn. Because they are Evergreen the green needles stay on and are exposed to the elements throughout the winter. Commonly, people will spray a product on the needles called an anti-desiccant. There are a few proven anti-desiccants such as Wiltpruf and Vaporgaurd. I mention these products because I trust them. Anti-desiccants can actually harm plants if done incorrectly or sprayed at the wrong time of year. These two commercial products in particular have been known to be pretty safe even with amateur gardeners. You want to spray the product when it is below 50 degrees but above 32 degrees. The anti-desiccant is essentially a wax and will only set it takes between those two temperatures. After spraying make sure there is a 24 hour period without rain, snow or any sort of precipitation to wash the product off.
In addition, you can put up wind barriers or windbreaks for little trees or shrubs. Of course this wouldn’t be practical for large and mature trees, which can protect themselves. This is for new plantings or for an immature plant that has not fully acclimated yet.
To start, drive four stakes into the ground, like you would for 4 corners of a box. Then take some burlap or something permeable, that breathes and then form a box, using the stakes as supports. The idea is that the tree is planted in an area where there is a constant stream of wind. That wind hits the branches that are furthest away from the trunk and wicks moisture out of the tips of the branches. It is removing the moisture thus killing the branch. The branch needs to maintain a level of moisture in it so it can stay plump and not dry out. So that’s why having anything to block a stream of wind is better than having nothing. It doesn’t need to be sophisticated or beautiful. You just want a layer to slow the wind down. This is very important for along driveways, sidewalks, in a front yard, anyplace that doesn’t have a lot of natural barriers to break the wind. The more open space you have the better chance there is of the wind drying out the plants in that area. This is a simple way to really reduce winter injury.
There are a few species that are common to our area in the Northeast that are prone to burns specifically, winter burn or windburn. It’s called a burn because it looks brownish black and there is no moisture left in it. Holly, rhododendrons, boxwood, arborvitae, spruce, hydrangea, and white pine are common trees to have in our area and unfortunately they are regularly injured in winter as well.
Prune & Protect
Prune all the suckers and dead branches. Again, refrain from fertilizing late because you will get a long tender growth and a cold spell that freezes it. Pruning in late fall combats this risk and encourages on an overall better looking plant in the spring. Next, remember to compost and mulch. Some people like to wrap the trunk with a permeable membrane. There is a specific tree wrap out there that is almost like a wax coated cardboard. This is mainly to protect from animals rubbing up against it and chewing on it when there isn’t a lot of natural, green vegetation for them to eat. Late fall and early spring there is not a whole lot of green out there, so squirrels, rabbits, and chipmunks go foraging and chew on the tree trunks. Many times they don’t eat it, but areare testing it, but this creates damage. Wraping the shrub or tree, you will give you one less thing to worry about.”