Gardening: Protecting plants in winter

Wrap up Warm“The simplest and most practical method of protection is placing the plants under a covered structure of some kind. If they are too large or if it isn’t practical to move them, then you have to bring the cover to the plants.”  James HartleyThere are scant few things in life on which one can depend unequivocally, and gardening is no exception. However over the years I’ve found horticultural certainties to abound, and they all tend to be rather perverse: You can count on it raining as you dig trenches to install a watering system (just as you do not need to check the weather forecast to know it will be particularly hot the day you dig a trench for a drain); equally, while you can count on grass growing in cracks in a path while it stubbornly refuses to grow in gaps in your lawn, you can assume that something unexpectedly thriving in a bed is a weed, yet a plant touted as “evergreen” will turn a peculiar shade of brown in the winter. Truth be told, winter stands head and shoulders above the rest with regard to its collection of inevitabilities, united by their involvement of cold. With an antisocial habit of announcing its arrival by way of a precipitous and unexpected drop in temperature, even the onset of winter is usually unappreciated by gardeners and their plants alike. For plants of a delicate disposition, good preparation is usually key to ensuring they make it through to spring in one piece.

Russian roulette

At the nursery we go to great lengths to make sure that clients are at least well informed about the risks they are taking with some plants, such as growing hybrid bougainvillea further than a stone’s throw from the coast which is nothing short of dicing with death. Despite this, since many of the plants that produce the best colour and do so for longest are those that cannot tolerate the cold, people just can’t resist the urge to take the gamble. If this sounds like you, you’ll probably have a large citrus tree planted somewhere nearby despite living nowhere near the sea, and as a consequence, you have a horticultural car crash waiting to happen. So – how to mitigate the inevitable consequences of a Siberian cold front sweeping across the region? Well, you’ll be pleased to know that it isn’t all in the lap of the Gods, and that there are good practical measures you can take to protect your treasured plants.

Hopefully, if you live in an area where frost is inevitable, you’ll have wisely planted your citrus and other frost-sensitive plants in containers. If this is the case, the obvious and best protection is to move these to a position under cover. Direct, or “radiation frost” is always a danger when the skies are clear, with the warmth remaining on the land’s surface after a sunny winter day being rapidly leaked away by radiation into the clear night sky. This effect is so dangerous due to the sheer speed with which the heat is lost: in many cases the temperature of the leaves of the plants will actually drop below the ambient air temperature as heat radiates away. As if this wasn’t bad enough, the effect is compounded as the sun bursts out the next morning, rapidly defrosting the leaves and in the process, bursting the cell walls within the leaves.

The simplest and most practical method of protection is placing the plants under a covered structure of some kind. If they are too large or if it isn’t practical to move them, then you have to bring the cover to the plants, and this involves large quantities of winter protection fleece. Winter protection fleece is a breathable fabric that helps to create a small microclimate in the canopy of your plant, keeping the temperature of the leaves several degrees above the ambient temperature, in particular under conditions that could lead to the reverse. The word “breathable” is crucial: never ever use plastic directly over plants as this allows moisture build up and rotting to start in the leaves and outer branches. The membrane sheets provide the protection required against heat radiating away and wind stripping the warmth from the leaves, whilst allowing moisture to permeate out and away each day.

On the subject of rotting leaves, it is well worth noting that fresh tender leaves are far more susceptible to frost damage than leaves that have been on the plant a while and have had time to harden off. To this end – make sure you ease off on the use of fertilisers containing high levels of nitrogen towards the end of the year as these will push plants into extended vigorous growth – leaving you with foliage that is badly unprepared for the ravages of a cold winter. If you are keen to keep your plants well fed, then opt for balanced feeds later in the season, or better still, feeds with elevated levels of potassium.

It’s worth noting that in these kinds of conditions it isn’t only tender and half-hardy plants that you have to keep an eye on. In extreme cases even evergreens and other hardy plants can be affected when the soil becomes frozen and the morning is particularly warm. Roots then find it impossible to lift moisture up to the plant and the plant can die from drought.

Winter can be cruel

It’s one of those inevitabilities that very cold weather will follow a windy period – something we have a problem with time and time again. The part we always have trouble with is when it has been particularly windy as this is when we can experience damage to our heated glasshouse or have winter protection removed. Then, with sadistic unoriginality, the temperatures plummet before you have time to react, and plants that have been kept tucked up warm are suddenly exposed to the elements.

In gardens this is also problem, and for those that have climbing plants protected on a wall, these are some of the most susceptible to having the winter protection pulled away by the wind. To avoid this, I strongly recommend using a particularly good method of attaching the fleece, taught to me when I was learning the trade.

Attaching fleece to a climber on a wall to stay up for an entire winter can be difficult to do effectively. This problem however can be solved elegantly with a couple of slats of wood. The strips of wood in question should be light, no more than about five or ten centimetres wide and a couple of centimetres thick, and should be as long as is required to be mounted vertically on the wall to match the height of the climbing plant you are trying to cover. Drill two or three holes (top, bottom and centre) in the strip, and hold the strip up to the wall on either side of the climber and mark the wall in the position of the holes. The wall can then be drilled and fitted with plugs to take screws. In this way you can screw the wood batons to the wall and use the wood to staple up the fleece. Starting on one side the fleece can be drawn over the plant, stapled on the other side and drawn back across in the same way a couple of times, stapling on each side, to create a protective layer that is both effective and reliably attached. Once winter is over, the fleece can be removed, and the wood strips taken down simply by removing the screws, allowing them to be stored and reused the following year.

A word of advice: be sure to drill the holes identically in each of the wood batons to avoids having to figure out in which position and in which orientation each piece of wood goes the following year. Accepted – this does have potential for a certain degree of yuletide horticultural hilarity, but you’ll just have to trust me. Invariably you’ll be doing this when it’s very cold outside (having misjudged the onset of winter), and the novelty value in such capers in indirectly related to temperature.

By James Hartley
Director, English Garden Group