With the increase in popularity of outdoor container gardens, people are spending more money on pots and on the plants they put in them. It's no longer a just a case of planting a few herbs in a pot, vegetables in a container or a row of flowers in a window-box. Serious money is being spent on fancy containers and specimen plants to enhance outdoor spaces. And it is understandable that people will want to protect that investment.
The arrival of more severe winters, or even surprise cold snaps are a much greater problem for container gardens than for traditional gardens with the plants planted in the ground.
Containers are exposed on all but the bottom surface to the effects of wind and ice. They are also limited in volume, so do not retain heat as well as a more traditional plant border.
There are two main problems associated with frozen garden containers. The first is that it can cause such severe damage to the roots of the plant, that they fail to survive into the next season. Although the same plant, planted more traditionally in the ground, might make it through the same winter.
The second big problem with frozen garden containers is that the containers themselves may crack.
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Frozen roots can mean dead plants
Freezing creates ice-crystals in moist plant tissue, and ice crystals destroy cells. If you put a fresh crispy carrot in the freezer, it will come out soft and bendy. It has lost important structure in its cells. Some plants have antifreeze down to a certain temperature but most overwintering plants can only overwinter because they have some adaptation to the winter weather. Perennials for example retreat under the ground, leaving nothing exposed to the elements. Trees and shrubs may lose their leaves and stop sap flow to their above ground parts to maintain life below ground level. Only a few particularly hardy plants can retain foliage and even grow in sub-zero conditions - all the others rely on their roots, bulbs or corms to carry them over till spring. But what they also rely on, is the fact that the deeper you go in the ground, the warmer it gets compared to a frozen surface. For example if you have 5 degrees of frost, you can easily scrape back the soil to find unfrozen soil beneath. Even at serious subzero temperatures, there will always be somewhere deep down that is unfrozen, and plants have become adapted to overwintering parts of their structure at that level. In a container a plant can only send its roots down to the bottom of the container, and the container itself is fully exposed to freezing.
The only way to help prevent containerised plants from dying of frost and frozen roots, is to provide them with some protection. Move containers indoors, or to a more sheltered part of the garden where they will be less exposed to chilling winds. Whatever the air temperature, winds can add a significant chill factor and remove the heat from your pots much faster, causing them to freeze solid at temperatures where the less exposed soil beneath the surface of a border will not freeze. If you cannot move the pot – perhaps you have no sheltered space to move it to, or your pot is too big to move, you can provide the soil in your containers with some insulation by wrapping the container in a few layers of insulating material such as bubble wrap. (You can buy horticultural bubble wrap in any good garden centre.) Providing a covered area for your pots can also help, as it prevents night frost falling on foliage, and stops rainwater soaking your pots which can cause water-logging and aggravate the freezing problem. Grouping pots together will also allow the pots to provide shelter for each other and if they are grouped somewhere that gets sunlight at some point in the day, they will heat up somewhat and retain that heat, almost to the point of creating a little area of warmer micro-climate in your garden.
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Freezing Weather Cracks Pots.
Cracking may be caused by two mechanisms:
- The water in the compost or soil in the container expands and forces the walls of the container outward, until they can take no more pressure and they crack. This type of cracking typically involves a full depth crack which may cause half the pot to fall away. 'Frost Proof 'pots are also vulnerable to this as it is purely down to the force of the water expanding in the compost or soil, and not in the pot. To prevent this type of cracking, keep your soil as dry as tolerable during the winter months of minimal growth. Make sure the drainage in your pots is adequate to deal with rainfall and do not water excessively. Make sure the soil only comes to a few inches short of any narrowing in the pot, as frozen soil being pushed into a narrower space has a lot more pressure to apply to to the wall of the container holding it.
- Water is absorbed by the material in the walls of the pot – typically terracotta, and when it freezes it expands in the terracotta and has nowhere to expand to. So tiny micro-cracks form in the structure of the pot. These develop into deeper cracks as the water absorbed into the walls of the container repeatedly undergoes the processes of thawing and re-freezing. This type of cracking is usually seen as flakes shearing off the outside of the pots. . Even glazed pots may be vulnerable to this kind of cracking as over time as tiny crackles in the glaze let moisture into the material beneath. Glazed pots, although they will have been fired to a higher temperature for glazing, are not fired hot enough to completely stop water permeation into the clay . So glazed pots which are unglazed inside will be subject to some water penetration. 'Frost Proof' pots are so-called because they are fired to a higher temperature for a longer period of time, to make the terracotta impermeable to water.
So choose your pots carefully, and look for a long freeze proof guarantee. And in winter remember that your container garden will require a different kind of maintenance than surrounding borders.