Written by davethetreecenters • November 14 Keeping Shrubs & Trees in Planters Alive Through the Winter

Many gardeners think no further than annual flowers when they think about planting containers. Others see it as a great way to grow shrubs and trees, especially if you have a courtyard garden, or you move a lot. It has another great use too – you can grow plants that aren’t hardy in your area, as long as you have a way to keep them through the winter. With winter setting in, this seemed like a good time to talk about this subject, and especially consider what it takes to keep these plants alive, so they can thrive, grow flowers and perhaps fruit, and become valuable specimens.

Why Planters and Pots?

If you thought that gardening was something that needed a patch of earth, think again. You can garden even if you have no soil, as long as you have an outdoor space. This could be a paved courtyard, a terrace or even a balcony – it’s amazing where plants can be grown. Doing this has become especially valuable with the trend towards greening our cities and making them more livable – you can do your part and enjoy the results.

Yes, it takes a little more attention, many to watering and also to fertilizing, but you will be amazed how much better plants do without competition, growing in a pot, compared to struggling in a difficult urban environment beneath trees, competing with them for space, water and nutrients – in pots they have their own private world.

Which Shrubs and Trees Will Survive Winter Outdoors in Pots?

This is a common question, and you can be disappointed when a plant that should be hardy in your area is dead next spring if left out in a pot. Why, you ask yourself? The reason is this – plant roots are not as hardy as top growth. It doesn’t need to be, because soil in winter is always a lot warmer than the air above it. There is a easy ‘rule’ though, which will mean success. Check the minimum growing zone in your area, then subtract two, and plants that are hardy in that zone will be fine in yours when growing in a pot. For example, if you live in zone 6, then plants need to be hardy in zone 4 to overwinter in a pot and be happy the next spring. Now sometimes you can reduce that by one zone – so zone 5 in our example, but that’s risky, depending on how severe the winter turns out to be, so start off with safe choices before pushing the limit.

If you follow this simple ‘minus 2’ rule, then you can just leave you trees and shrubs outdoors all winter, no problem.

What About Less Hardy Plants?

Now, what if the plant or plants you love and want to grow are not hardy enough for that? Or maybe you want to grow a fig tree, but live in zone 4. There are some strategies for keeping these plants through winter, but bringing them into the house isn’t one of them – unless it’s a real houseplant, like a Ficus or Yucca. The problem is, your house is too warm. Plants that grow outside of the tropics need a winter period of cold, generally below 45 degrees, to properly mature and develop both their flower buds, and their leaf buds too. This is especially true for fruit trees, and it’s even true for citrus fruits – oranges and lemons. It’s also true for camellias and azaleas too, both popular plants for pots in areas where the natural soil isn’t acidic.

The answer, therefore, is to keep them in places that are below 45 degrees, but ideally above freezing, although many can take 5 or 6 degrees of frost without problems.

Overwintering Tender Deciduous Trees and Shrubs

If you plant you want to grow is deciduous, overwintering is relatively easy. If the plant is hardy for your winter, but not 2 zones hardier, then you can also try wrapping the pot in Styrofoam, being sure to cover the soil area, but not the bottom. If you can stand it directly on earth, then it will draw some warm from that, but up in the air or on concrete it probably won’t do so well. Try this if you don’t have a suitable storage space.

Better, though, is a space that is cool enough, but not cold. You don’t need light – no, you don’t, not if the tree is naturally deciduous. This could be, for example, a fig tree, an apple or peach, or a deciduous azalea (yes, they exist and are very beautiful). It could also be a tree rose, or any flowering shrub that won’t make it outdoors for you. The storage space could be a cold garage or shed, which is often ideal, especially if it is attached to your house, getting some heat through the walls or doors, and so won’t become too, too cold. You might also have a basement that is cold enough – check it with a thermometer during winter and see.

Keep the plant outdoors until it drops its leaves and temperatures are getting close to freezing, and then move it in. Put it out again as soon as it isn’t bitter cold, because there is a danger, if the space heats up as the weather warms, that it will start to sprout in the dark. You can reduce the chances of that by letting the soil become dry – not rock dry, but much drier than normal. Since there are no leaves, no harm will be done. Once you bring it outside and water it will take off, and hopefully flower and fruit as you wish it to.

Overwintering Tender Evergreen Plants

Citrus, camellias and azaleas are the most popular plants in this category, but you might have other favorites. Doing this is a bit trickier, since these plants continue to need light, or they will simply drop all their leaves. A cool, bright space is needed, such as an east or south facing window. Perhaps you have a room you don’t use much in winter that has a bright window? Can you turn down the thermostat to 45, and keep the door closed?

Best of all is a glassed-in porch or sunroom of course. In some areas there is a danger it will get too cold, but a simple fan heater, attached to a thermostat, is great insurance, and one could also be used for those deciduous trees you have in a shed. Set the thermostat for 34, and again it is best to keep the plants on the dry side, but of course not as dry as you can with storing deciduous plants.

How Many Weeks of Cold are Needed?

This is a good question, but a tricky one. Growers of fruit use the concept of Chilling Hours – discussed in detail in the linked blog. If you are buying fruit trees from us, you will normally find this number in the detailed description. Otherwise you can often find it online. For flowering trees and shrubs there is much less information available – we often simply don’t know. A good general rule, though, is to go with 12 weeks below 45 degrees. Some things need longer, but for citrus and camellias, that will be plenty. So if you bring your plants in at the end of November, you can expose them to higher temperatures from March on, depending on your exact situation. If you have that glassed porch or sunroom, those plants could easily bloom earlier, they will decide, and you will love having them in bloom, safe from cold spring weather. Move them outside when things get warmer.