The clocks have changed, the nights are shorter, and much colder, and maybe there have already been flurries in the air. As winter comes our gardens go to sleep, but if you garden in containers, this isn’t yet the time for you to do the same. It’s the lucky gardener who lives in a warm zone who can leave their planters to take care of themselves overwinter – everyone else needs to take some steps to protect them. We are increasingly using shrubs and perennials in planters, and many gardeners want to keep them from year to year. Let’s look more closely at this tricky issue.
What Chance Do My Plants Have?
If you have shrubs and perennials growing in planter boxes, they are not going to be as hardy as when they grow in the ground. Why? Because plant roots are never as cold-resistant as the branches. They don’t have to be, since soil never reaches the same temperature as the air, it is always less cold (and less hot too). Even in places where the ground freezes hard, it never approaches the chilling cold of the air above it.
As an aside, it’s worth pointing out that since plants are ‘cold-blooded’, they aren’t affected by wind-chill. The real temperature is all they feel and those ‘feels like’ temperatures only affect us, not our bushes.
There can be exceptions, but as a ‘rule of thumb’ we can add two hardiness zones to a plant to know if it will survive the winter in a pot. So if your plant is hardy to zone 4 it will easily survive in a pot in zone 6, less likely but worth a go in zone 5, and not at all in zone 4. If you have shrubs in your planters and pots the first step is to check this out. If they are inside their ‘comfort zone’ you have a good chance of them surviving. If their hardiness is the same or less than your zone then leaving them in the pots is definitely not going to work – we’ll talk about a solution further down.
Contact With the Ground is Important
Since the earth is warm, it can make a difference if your pots are standing on it, especially in that zone one warmer than the coldest for the plant. It will help if you move your pots onto the soil, and this is why plants on balconies are especially hard to keep overwinter – they get colder than ones on the ground.
Although there is some benefit if the pots are large, if the periods of cold are short, once you have several days of deep cold, the temperature will be the same all the way through the pot, so standing them on soil helps, no matter how big the pots are.
How to Winterize Your Potted Plants
Wrapping Pots – using bubble-wrap and garden fleece around pots will help if you are in a borderline situation with plants that are close to, but not in, the ‘comfort zone’ mentioned earlier – 2 zones warmer than their minimum zone. It’s not a guarantee if there is an extended period of deep cold, but it often makes the difference.
Wrapping also can help protect the pots themselves from cracking, although that can be tricky with terracotta (red clay) pots. These absorb water and that then freezes inside the porous structure of the clay, expands, and splits the pot. If you have valuable ornamental terracotta pots, it is best to transplant into cheap plastic, or the ground, clean the pots and store them somewhere dry – only damp pots split.
Spraying with Anti-desiccant – these all-natural products, made from pine, create a protective coating over evergreens and the buds of deciduous plants, reducing the risk of them drying out and dying back. They can be especially useful for plants like boxwood and cedar in pots. Since you have them mixed up, spray your garden evergreens too, especially newly-planted ones – you will be amazed at the benefits of this under-used treatment.
Wrapping the Plant – I don’t ever recommend wrapping plants for winter protection. It usually does more harm than good. Plastic is worst of all, as it suffocates them and traps too much moisture. Burlap is better, but it needs removing early, in late winter once the worst of the cold has gone. If you leave it too late it traps the heat of the sun, encouraging buds to open too soon – these are then prone to damage from frosts.
The only type of wrapping I have ever used is netting. This is good for evergreens, as it holds the branches together, slowing air-flow through them, and it protects against snow and ice opening up a plant, damaging the look and often causing branches to tear away.
Bring Them Indoors – some plants obviously need warmer conditions – a citrus tree for example. You can bring these kinds of plants inside your home, preferably in a cool, well-lit place. A slightly-heated porch is often better than a hot living-room. Keep them on the dry side, and keep them outside for as long as possible in fall, and soon in spring.
If they are deciduous plants – a fig tree for example – they don’t need light, and they can go into a garage or shed. Again, cool to cold is best, as long as it is above freezing – 40oF to 45oF is ideal. Many deciduous plants need winter chill to grow properly, so kept in a warm place they won’t go dormant properly, and never flower.
Transplanting into the Ground – This is the best and most reliable solution, especially when you are no more than one zone away from a plant’s minimum. If you have room in your garden you can simply dig a hole and drop the pot into it, or slide the plant out of the pot and ‘plant’ it without disturbing the root ball. Put the soil back as if you were planting, and water if necessary, especially for evergreens. This also means you can dry and store your pots safely, protecting them from cracking. Once the ground has thawed in spring you can plant back into the pot, using some fresh potting soil if necessary. You can usually keep a plant alive in its coldest zone if you plant it in the ground for the winter. Yes, this is the most work, but it gives the best results.
Throw them Away – I kept this to the last, as it can sound harsh, but if you don’t have the ability to protect plants in pots with any of these methods, throw them away in your fall cleanup. If they are right outside their hardiness zone, or you are on a balcony with no access to a garden, then it’s better to not waste time and effort and just replace them next spring. Sorry!