Winter Protection & Cold Hardiness
Growing exotic plants outside in the UK is rapidly becoming a popular subject with our changing climate, the often warmer, longer summers and generally shorter milder winters. Due to these changes we are finding more and more tropical to sub-tropical species are becoming quite capable of surviving the Great British weather. Despite these changing environmental factors tropical gardening in a temperate climate does have it's pitfalls and disappointments, but we feel and think, this is half the excitement. Hopefully this little guide based on our own experiences over many years of growing and experimenting will help you on the way to creating that little bit of tropical paradise in your own garden.....
The first thing, and we feel is quite a biggy, is a plants provenance. Where it originally comes from, how they have been grown, what extremes have they been exposed to and for how long, as well as other variables. This all makes a great difference to how a normally half hardy to tender plant will survive outside in what can only be classed as our quite unpredictable weather system.
The origins of the original stock plant, plants or seed has great importance, let’s take Brugmansia sanguinea for example. A half hardy large shrub to small tree endemic to the Andean mountains, with a distribution from Colombia to northern Chile where they can be found growing in elevations from 1000mtrs to 3000mtrs above sea level. Seed or cuttings taken from the plants at higher levels and from Chile would have most likely experienced greatly cooler conditions than the plants growing at lower elevations in Columbia and are more likely to be hardier than low elevation plants.
Now it is not always possible to source plants from cooler parts of their natural range as sometimes they do not naturally occur in or have cooler ranges, so we have to make do with what we can source and then do a few years of hardening off ourselves. This does not work with all exotics (disappointments abound with quite a few species) but does with many and we are finding more and more tropical plants are a bit more cold hardy than often thought. Currently we are having surprising success with some Calathea species and relatives, many of which are said to require a minimum temperature of 15c. This is not proving to be the case and some of the varieties we are playing with are surviving temperatures down to 0c so it should be said exotic gardening is often a rule breaker!
Another provenance factor is how the plants have been grown. Plants grown in a fully heated, hi-tech, artificially lit Glasshouse, where growing conditions are kept optimal all year round will not be as hardy as plants grown for several generations in cooler glasshouses, where temperatures are allowed to drop below a plants usual limits and where daylight levels are lower during the winter months. Okay, the plants grown in artificial conditions will be looking stunning in the middle of winter but as soon as the cold hits them it often becomes another story, a story from which they will rarely recover. On the other hand, an exotic that has been gradually exposed for numerous seasons to less than ideal conditions through the winter may not be looking at its best from November to March (and may even be dormant, which is not always natural for them) but the likelihood of it recovering during the warmer months is much higher.
You may or may not have noticed that sometimes a neighbours garden may have a frost on their lawn during the winter when yours does not or vice versa. This is because many gardens can and do have their own micro-climate, not only their own micro-climate but also micro-climates within the micro-climate. There are so many variables that can create or change the micro-climate in a garden and below you will find a few examples but the key is to really get to know your garden… Take notes and keep records especially during the colder winter periods.
A gardens micro-climate can vary greatly, even within the space of a few feet. The Canna musifolia we have growing under a Eucalyptus tree can often remain in leaf well into the winter before a heavy frost knocks them down to the ground yet the same Canna species 4 ft in front of the Eucalyptus gets knocked back at the first sign of frost. This is because the spreading canopy of the tree acts as a natural duvet and creates a warmer climate, but the front of the bed, where the canopy does not cover, is open to the elements so gets exposed to the earlier frosts. Raising the canopy/ crowns of mature trees and shrubs can create ideal warmer micro-climates beneath for your exotics, just make sure the crowns are raised enough to allow adequate enough light penetration. Although planting under evergreens is often ideal even the branches of a deciduous tree can make the difference between the ground freezing or not freezing in winter. Growing under the shelter of hardier plants will often give greater overwintering results than a tropical planting in the middle of an open, exposed, lawn bed.
The area your garden is in is also of great importance, the West Country is generally milder than the South, the South generally milder than the South East and so on and so on. Inner city gardens are usually milder than rural gardens with town courtyard gardens surrounded by other buildings often never even seeing a frost due to the radiated heat from the buildings. This opens up a whole new world of what can and cannot be grown outside during our winters. A garden out in the sticks (especially if exposed and having no surrounding wooded shelter belts) will often suffer heavy frost limiting your options but planting hardy trees and shrubs can create warmer micro-climates extending your options. A garden at the top of a valley will remain warmer than one at the bottom of a valley as cold air sinks and warm air rises. Coastal gardens (although often windy) rarely see frost so a sheltered area on the leeward side of the house can be an ideal spot for experimenting with less hardy plants.
There are many other factors to consider such as walls. South facing house walls usually have a milder climate than an east, west or north facing house wall. A house wall will be warmer than a freestanding garden wall. A house wall with a boilers hot air vent poking out will create an even warmer micro-climate. A wall with a patio canopy (although open sided) will trap warm air keeping the area milder during the winter. There are lots of little things that can make the difference to what will and will not survive a winter in your own garden.
It is not the cold that is so much of a problem for many exotic/ tropical plants, in fact, as long as the roots do not freeze many warmer climate plants are reliably root hardy. It is the winter cold and wet combined that is usually the killer. There are many species of Cacti and Succulents as well as Mediterranean plants that are more than capable of tolerating low, freezing temperatures as long as the compost/ soil is dry or relatively so. If the roots are sitting in a cold wet compost then that can be big problem. Although they may experience night time temperatures down to say -5c in the wild (Quite common in some deserts), the sandy, rocky, gravely ground they are used to will have little in the way of moisture. This means the ground itself will not freeze and will warm up rapidly during the day. Wet soil will not warm up during the day especially in our winters and it will often freeze solid. Both of these factors can cause rot to set it killing the plant from the roots upwards. So, for hardier species of cacti and succulents a very free draining, open soil is a must.
Hardier Palms, Canna, Hedychiums and other rain/ cloud forest type plants will usually require a well drained but humus rich soil. Again it is the cold and wet combined in winter that is the biggest killer, especially if they are sitting in cold, stodgy clay. If you are unlucky enough to have a poorly drained clay soil then lots of mulching with compost, bark chippings, horticultural grit and fine gravel will be required for successful in ground overwintering outside.
Wrapping and Protecting.
Whether you will or will not have to provide your exotics with some form of winter protection will depend entirely on the plant, your location and how hard a winter is expected. There are many techniques for various species. In coastal parts of the South of England plants such as hardy gingers, canna, dahlia and other hardier exotic rhizomatous, tuberous, bulbous plants will usually survive an average winter left as they are in the ground, dying back and going dormant until new growth starts in late spring. In colder areas all they may need is a thick mulch of bark chippings to reduce the risk of the ground freezing. In areas where the ground usually freezes solid or during the rarer extremely hard winters they may need chopping back, lifting and storing in slightly damp peat in a dark, cool, but frost free shed or garage until re-planting in the late spring.
Less hardy palms such as Phoenix canariensis may need wrapping. Again, depending on the winter and area this may only need doing with younger plants until they have built up some cold resistance. In milder areas this may be nothing more than pulling the fronds up and tying them in so they look like a closed umbrella. The outer fronds may get damaged but it will give some level of protection to the inner fronds. In colder gardens it may involve protecting the crown with straw, pulling up the fronds and then wrapping with horticultural fleece. Do not use bubble wrap, it will not breath, it will sweat and cause moulds and mildews.
Straw is an exceptionally good insulator if kept dry and can be used in various ways for various species. Hardy bananas (Musa basjoo and others) will often die back to the ground during very cold spells. Although they are reliably root hardy they will only reach around 6 to 7ft in height each season if having to re-grow from the rootstock. If you want really big plants you need to maintain the trunks through cold spells below -4. The best way to do this is using straw and something to hold the straw in place and keep it dry. There are several methods but we use a thick grade, fine holed, plastic mesh and an old piece of carpet. The trunks are packed around with straw and then the carpet and mesh are curved around and tied into place creating a cylinder. More straw is packed in and once it feels sturdy is capped of with a slate tile to stop excess wet getting into the straw. This is then removed around mid April. Again do not wrap with bubble wrap or plastic sheeting. It will cause too much humidity and various moulds and fungi. This technique can be used with many other exotics.
Another technique which we regularly use and have found highly successful with many tropical shrubs and perennials is cloches. A standard bell cloche, tunnel cloche or mini, temporary greenhouses are not always adequate enough on their own but with a little adaptation can make the impossible possible. We have successfully overwintered Bouganvillea, Lantana, Ricinus, Citrus, exotic ferns and many tropical perennials with this technique. If the cloche has a top vent make sure it is closed, you will need ventilation but you do not want much, if any, water getting in. Cut back the plant to fit the cloche and lightly pack around the plant with straw creating a nice, breathable duvet. Then, place the cloche over the plant raising it up slightly from the ground on some broken, slabs, wooden blocks or bricks. This will allow ventilation but at the same time keeping the straw relatively dry. Remove the straw and cloche once all heavy frosts have passed and away you go.
Cloches can also be useful for keeping hardier cacti and succulents dry for the winter. Again keep the top vent closed but raise the base up a bit to allow air circulation.
For those of you that want to create a jungly or tropical style garden but really are in a part of the country that gets colder than average winters or you just don’t want the hassle of wrapping and protecting plants during the colder months then do not despair. There are many hardy plants that can be used to create a similar effect. There are large leaved Hosta, many types of fern such as Woodwardia fimbriata, numerous species of trees, some of which will give you giant leaves if coppiced each year (Paulownia, Catalpa and Juglans to name but a few), hardy hot coloured perennials and bedding annuals as well as a large selection of shrubs with contrasting foliage and tropical scented flowers. Even some of our own natives such as Asplenium scolopendrium can lend a hand. Exotic does not necessarily mean it comes from a tropical climate or is not hardy, it just means it is not native.