Pushing the Envelope on Cold Hardiness

This winter in the Pacific Northwest was a good reminder that those low winter temperatures are still possible after a few years of having extremely mild winters. For lovers of subtropical or frost tender plants pushing the envelope on cold hardiness can be a stressful time as you watch the mercury drop on the thermometer and hope for the best as your cold hardy citrus, Chilean guavas, or avocado trees get covered in snow.

There are many factors that play into how well a plant will do in a specific area.  Climate, soil types, chill hours, heat zone, elevation, latitude, slope orientation and many other factors determine whether a plant will thrive in an area, but one of the most obvious limiting factors that can decide if you’re able to get a certain plant to grow in your area is its cold hardiness. If your winter temperatures get too low for a plant to survive then it doesn’t matter what kind of amazing soil amendments you’ve added because the plant simply will not survive. For many plants, such as those of tropical origins, there is not much we can do aside from building a year round fully heated greenhouse or bringing them inside our heated homes. But for plants that that are right on the cusp of their cold hardiness boundaries there is plenty we can do.

The first and most obvious technique for pushing the envelope on cold hardiness is to mulch heavily. A plant that has a thick layer of mulch laid down in the fall before the cold of winter sets in will have an insulating blanket of organic material around its roots that will moderate soil temperatures and protect the root system from deep freezes. Snow is also an excellent insulator so if you live in a climate that receives regular snowfall don’t worry about removing it from the base of your trees. It is actually helping to keep the roots insulated.

The next technique that most people focus on is planting in micro climates that are slightly warmer than surrounding areas. The south facing wall of a house is a popular location in many urban areas and while this technique certainly works for providing extra warmth and heat to species that need such conditions to ripen their fruits it may or may not help in overwintering your frost tender plants. It depends on the climate and the plant in question but if temperatures are above freezing during the day but dropping below freezing at night it can be damaging to have your plant in a location where it will heat up during the day but then freeze rapidly again at night. This freeze thaw cycle can not only damage the vascular system of a plant but actually cause them to explode! Well maybe not explode, but when the bark splits it often sounds like an explosion when it happens. On clear winter days when the sun is shining bright on the dark bark of trees it can cause them to warm up and expand the wood beneath the bark and cambium. When temperatures drop quickly again after sunset the sap will freeze as the wood contracts and it can cause huge splits in the bark that sound like gunshots going off. This should not deter you from utilizing these south facing microclimates, just be aware that it doesn’t work for every species or every climate.

For broadleaf evergreen trees such as citrus or pineapple guava the deep cold can damage the vascular tissue of the leaves.  To combat these deep freezes many gardeners have taken to putting Christmas lights on their trees and turning them on whenever frost threatens. It is very important that you use the old fashioned incandescent bulbs as the new LED ones don’t give off significant heat. You can also build a custom hoop house or even wrap the tree in plastic to insulate it and keep the heat from your Christmas lights in.  We’ve seen people overwinter very tender perennials such as Meyer lemons using this technique here in zone 7 and 8 through frosts as low as 5 degrees Fahrenheit.

Placing your frost tender tree underneath some sort of frost blanket will also help it to avoid at least the light frosts. Pay attention on your first frost date of the year to where the frost settles. There will often be pockets where it doesn’t freeze underneath the canopies of large trees as the frost settles up in the tree rather than on the ground. Placing trees on slopes that drain the frost well rather than in low places where it gathers can also help tremendously.

A sun trap is another useful technique that places your tree in a sunny south facing (here in the Northern Hemisphere) location and places rocks, bricks, or organic material around the outside of the plant in a U shape with the opening of the U facing south to act as a thermal battery as it collects warmth during the day and releases it at night when colder temperatures threaten. Creating the sun trap so that it also blocks cold winter winds or planting some sort of windbreak will also help.

The final technique we’ve found that works well is planting trees that have a slightly thicker caliper. The “rule of thumb” is to wait until the main stem is at least as thick as your thumb. We’ve seen cold hardy citrus trees lose their tender young shoots during very cold winters here but the plant survives because the main stem and the more mature woody branches are not damaged. This might mean you have to bring your plant indoors for the first winter or two before it is woody enough to plant outside. Planting in the spring rather than the fall can also have a slight added benefit as your tree will have the entire season to get rooted in its new location.

It may seem like a lot of work to take all these extra steps to get your plants to survive and while we usually recommend people plant things that are best suited for their climate it can be fun to push the envelop on cold hardiness. It forces you to pay attention to your winter temperatures, the way weather pattern move in your area and the historical low temperatures for your climate. All the Christmas light hanging and tree wrapping becomes immediately worth it when you find yourself with an abundance of fresh, pesticide-free, rare citrus fruit in the middle of a northern winter.  Your neighbors will quickly want to befriend you when your homegrown limoncello is ready in the middle of winter! And finally, in a time of climate chaos we find it wise to plant for every possible future climate scenario. Plant trees that will survive temperatures well below your record low temperatures as well as trees that require a much warmer climate to really thrive.  The potential for fresh citrus, loquats and avocados is too delicious not to! Either way it’s more interesting than planting another boxwood or rhododendron.

As always we love to hear our customers’ stories about any techniques they’ve found that have helped them overwinter a tender plant or grow something where everyone else said it couldn’t be grown. Write us at info@onegreenworld with your stories and techniques!