Whilst winter is a still (thankfully) a couple of months away, now is the time to get perennial plants ready to face the harsh conditions and ensure they survive, or even thrive, in the winter season. This is especially true for any half-hardy varieties, potted plants, or younger plants. However, even the most resilient conifer can still suffer damage in winter if unnatural conditions knock their internal dormant phase out of kilter. So here are Eutrema’s top tips for producing hardy growth in autumn:
Reduce growth rates
The first thing you should do is stop applying nitrogen fertilisers. This is for two reasons. Firstly, nitrogen stimulates excessive vegetative growth of stems and leafy tissue. This growth is luscious but weak and sensitive to frosts. Secondly, the microbes in the soil that convert nitrogen into useful chemical forms can stop working at low temperatures. However, the microbes that convert nitrogen into non-useful/toxic nitrites, do not stop working in low temperatures. So excess nitrogen in winter can actually poison the plants.
The second way to reduce growth rates is to start to implement a deficit irrigation regime. Reduce irrigation levels to at least 75% of total potential evapotranspiration. Not only will this reduce the growth rate of stems, it will promote root proliferation, improve floral bud dormancy, and the ABA stress hormone produced will lead to chemical changes in the cell that will aid dormancy in extreme temperatures. All of these combined will help winter survival.
Strengthen cell walls
Whilst cellulose and lignin make up the vast majority of a plant cell wall, deposits of calcium (in calcium pectate) and silicon (silica) are also added to further increase their strength and thus overall plant hardiness. This is especially important for plants growing in compost or coir that lacks calcium. Furthermore, standard ‘complete’ fertilisers lack calcium and silicon in the UK, so you cannot rely on your fertiliser to supply these. Try adding a silicon and/or calcium supplement feed in autumn if your current fertiliser regime lacks one or both of these nutrients.
Strong cell walls are required to resist many environmental stresses that plants encounter, but are especially important when it comes to cold and frost tolerance.
Freezing temperatures damage plants on a cellular level, with ice crystals forming first in the spaces between cells. This ice damages cells in two ways; it can puncture cell membranes and as the ice forms it can draw water out of the cells.This leads to the dehydration of the cells, which is why plants often wilt in freezing temperatures.
Insulate the roots
Mulching the surface of the soil/growing medium is great for innumerable reasons, and one of these is insulation from extreme temperatures. In winter this means protection from deep frosts, but it also means protecting the roots from warming up too early, which pushes resources into the buds. They then get hit by a late frost. Both of these conditions can be fatal. A common symptom of a loss of acclimation too early is scorching of the foliage in spring, which is a particular problem on newly planted bare-rooted trees and shrubs.
Put the secateurs away for the winter!
In certain plants, if you want to produce an extra flush of growth at the end of summer you can circumvent the dormancy of stems by pruning or punching the buds off. This is an especially effective technique for plants that would normally be forming flower buds in their apical buds in late summer and it can be used to generate new growth for use in softwood cuttings or to obtain a large established shrub in less time. This extra flush of growth uses the reserves of energy that were being stored for floral development, flower opening, and seed and fruit production the following spring. However, stimulating an extra flush of vegetative growth too late in the growing season could result in the new growth not being fully acclimated in time for winter and so this soft herbaceous material often dies off if there is an early winter frost. **This loss of hardiness was found in leylandii hedges, with studies in the USA in the early 1990s finding that an autumnal pruning reducing the plants’ winter hardiness by 2-6°C.
Make sure the plants are not illuminated at night
Some species of plants will not enter dormancy if they are exposed to unnatural photoperiods from artificial light. These plants are then extremely vulnerable to frosts. If trees and shrubs are not fully dormant before the onset of early winter frosts then the tissues will be subjected to winter frosts in an un-acclimated state. For some species the reducing daylengths of autumn interacts with a decreasing temperature to signal and control the acclimation process that precedes dormancy. Problems with plants not entering dormancy are sometimes found in trees, such as empress tree (Paulownia tomentosa) and black cottonwood (Populus trichocarpa). Therefore, these susceptible trees should ideally not be planted close to security or street lighting. The worst case of this I have seem is the alder trees planted on Bond Street in London that still have a canopy of fully green leaves at Christmas time, presumably due to the high intensity of the street lighting on this busy shopping area.
Choose your plant species and cultivars to suit the climate
Severe winter frosts in Britain will also damage or kill a number of half-hardy shrubs like Oregon grape (Mahonia aquifolium), four o’clock flowers(Mirabilis), Griselinia littoralis, New Zealand flax (Phormium), Escallonia, Cordyline, Viburnum tinus, Bomarea and Ceanothus. In northern Britain, varieties of green bottlebrush (Callistemon viridiflorus), crimson bottlebrush (Callistemon citrinus), Jerusalem sage (Phlomis fruticosa), Chinese lantern (Abutilon), and even privet may also be killed by prolonged periods of severe frosts.
But at least you aren’t growing the Brazilian cactus Coleocephalocereus fluminensis which cannot survive below 20°C!
Wrap up semi-hardy plants during prolonged frosts
Some herbaceous perennials do not die back to their root system in winter and are termed ‘wintergreens’. Examples include bananas, Fuchsia, Phygelius , and tree ferns and they are particularly prone to winter damage/death. You cannot get away with leaving these plants exposed to the elements in the winter; even with climate change producing milder winters. Whilst you will not be able to prevent a plant growing in the ground from experiencing frosts in the UK, you can prevent the harmful effects that result from ‘wind chill’. Wind chill is an effect of low temperature and air movement that most horticulturists know about but do not realise affects plants as well as humans. The main problem with wind chill for plants is the increased desiccation from freeze drying of tissues in sub-zero winds. Additional problems of winds can include it removing snow from around ground plants where it would normally insulate them from extreme frosts, and wind chill.
Materials that can be used to insulate plants from winter wind chill include:
- Bubble wrap is a very good material for insulating glasshouses and it can also be wrapped around tree ferns and bananas planted outdoors. However, extensive use of plastics is not environmentally friendly and the plants can get very moist inside and thus can succumb to rotting due to the poor air flow inside the material.
- Household insulation, such as rubber carpet underlay, can be reused to encase another insulation material and can be wrapped securely around the plant.
- Straw can be placed around stems, and secured with wire mesh, but straw is prone to waterlogging and thus can lose its thermal properties. Therefore, it is best used when encased inside a plastic cover, such as old grow bags.
- Untreated greased sheep’s wool is a sustainable material and can keep the frost out if wrapped around plants while still allowing some air flow. Unlike many other natural products, untreated sheep’s wool does not hold much water due to its lanolin content. Again, this can be encased with spare plastic sheeting, which will have the added benefit of reducing the wind-chill experienced by the plants.
- Breathable fabrics are materials that are normally used to waterproof/damp proof large buildings, however they can also be used in horticulture. The advantage of using breathable fabrics (e.g. Tyvek) is that water lost by the plant through transpiration inside the enclosure can escape through the pores in the fabric and thus minimise damp conditions inside the enclosure. As the fabrics provide little insulation themselves they need to be combined with a thermal insulator such as straw or building insulation. If stored over the summer these fabrics should be able to be re-used each year.
If you have the time and man power, try to remove the wrapping when mild conditions (no nighttime frosts) are forecast for a number of weeks. This will help to prevent the conditions that could lead to rot setting in.
By following these guidelines, you can safeguard your perennial plants, ensuring they emerge from winter’s embrace stronger and healthier in the coming spring. Remember, a little preparation now can go a long way in preserving the vitality of your plants throughout the colder months.