Using shrubs in planters and pots is a great idea, and widely suggested, not just for anyone with limited garden space, but also in larger gardens, to decorate terraces, frame garage doors, or along driveway. They look great, and they really add a whole extra dimension to your garden, so its really is a wonderful gardening trend.
You enjoyed them all summer, but with the arrival of cooler nights you might be looking at those planters with some concern. What is going to happen to the shrubs in them, that have done so well, and that I want to plant to keep for years? Your concern is a genuine one, because all too often we go out into the garden in spring, only to find those lovely shrubs dried and dead, either completely or in part, so they no longer look like they did, and are eyesores, not eye-candy.
Roots are Not as Cold-hardy as Branches
You carefully chose plants hardy in your zone, so what the heck went wrong? Many gardeners don’t understand that the root systems of plants are not as hardy as the top growth. In a very real way, the roots live in a different hardiness zone from the branches. Everyone has seen how the air temperature can be well below freezing, yet the soil is only frozen for the top inch or so, and deeper down it can still be warm, especially in early fall. There is much less push from the environment for plants to evolve very cold-hardy roots, so they simply didn’t do it. Roots also naturally continue to grow at lower temperatures, and they are often active in late fall and early spring, when the top growth is too dormant.
The New Jersey Agricultural Experiment Station of Rutgers University has published some helpful research on this. For example, they found that the saucer magnolia, Magnolia soulangiana, which is a popular spring-blooming deciduous tree, hardy to zone 5 and often 4, has roots that will be 50% killed when the soil temperatures reaches 23 degrees (F) – and that is zone 9. The popular Japanese Cedar, Cryptomeria japonica, is hardy to zone 5, but the roots are 50% killed in soil at 16 degrees (F) – zone – zone 8. Lots of people love to grow clipped boxwood in pots, and they are typically hardy in zone 5 in the garden. Take a look at the roots, though, and they are badly damaged at minus 9 degrees, which is only the cooler end of zone 6.
Some plants do a little better. The always-loved Blue Spruce, Picea glauca, doesn’t suffer from much root damage until the soil is minus 23 (F), which is zone 4, but remember that this is a shrub that is completely hardy all through zone 3, with air temperatures as low as minus 40. Creeping Junipers (Juniperus horizontalis) too, known for the hardiness, are damaged in soil that is only minus 18, zone 5, although these plants are perfectly hardy in zone 3.
How Serious is This Problem?
This doesn’t mean the plants will definitely die – with some roots still alive they can often recover, but it may be at the expense of some foliage, or whole branches. By the time they do recover, winter will be back, in after a few years of this cycle they will often decline and die completely. There are other mitigating factors too, such as the thickness of the walls of the container, if they are insulated, and if they are in contact with the soil. Plants on decks and balconies are much more likely to die, because they are not in contact with warmer soil at all. We can start to see a pattern with these examples. While there are some exceptions – the magnolia for example, it seems that roots are between one and three growing zones less hardy than top growth. We can see an easy ‘rule of thumb’ emerging here. Although there can be exceptions to it, we can probably say:
For container growing, stick to shrubs and trees two zones hardier than your growing zone.
Although there area few ‘toughies’, like the Blue Spruce, that can handle just one zone difference, and some ‘wimps’, like the magnolia, that need 3 or 4 zones difference, this rule is usually going to work well, although balconies are still tough, and need some extra care. With that rule in hand, we can go out and choose shrubs and trees for planters and boxes, and that way you can be almost certain they will be in perfect health the next spring, and go on to thrive for many years.
But I Live in a Cold Zone – What Can I Do?
Of course, if you already live in zone 3 and 4, you can see immediately your choices are very, very limited, and almost non-existent. What to do?
There are a couple of easy solutions. The simplest is to grow the shrubs for large planters in their own individual pots, placed inside the planter, with our without the surrounding spaces filled with potting soil, mulch, bark, clay pebbles, or some other material. Then in late fall, before it starts to get too cold, lift them from the big planter and place them in the garden. Simply standing them on the ground will probably work if the growing difference is just one zone, but it is safer to dig a shallow trench and bury the pots at least one-half into the ground – completely buried is better, or you could cover the upper sections with a thick layer of mulch, which is a lot less work. If you did plunge the pots into some kind of medium, like mulch, in the bigger planters, they will probably have rooted into it through the bottom, so you will need to do some trimming.
An alternative is to buy a long, narrow spade – they are often called ‘transplant’ spades’ – and neatly extract the sensitive shrubs from the planter boxes with it, and then plant them in the garden for the winter. This is usually fine for deciduous plants, but perhaps not so good for evergreens, where the ‘pot in pot’ approach works better.
Don’t Forget to Water
Remember to water those plants well when you move them into the garden – and that brings up something else too. If you have evergreens in planters, be sure to water them deeply in late fall, because if the soil is dry, they can suffer winter burn of the foliage, as they continue to lose moisture into the air in winter, especially when cold, dry winter winds are blowing. You could easily need to water again as winter goes on, so keep an eye on them.
That’s It . . .
This principle is often forgotten, but now you know it, you can look forward to having great winter survival of your plants in containers – a little knowledge makes gardening a whole lot more fun, and more satisfying too.