Heading into Winter with Exotic Plants – October 2021

In August 2020 I treated myself to a pair of standard lemon trees at a bargain £40 for the pair. They overwintered well in my unheated greenhouse and have since flowered and fruited magnificently. However, I have since realised that there is a problem brewing... they are not grafted onto rootstock suitable for container growing and are normal lemon trees that will want to break free of their pots and our modest greenhouse before too long. In a non-Mediterranean climate, this is a problem. To some extent I can prune the top growth- but the roots cannot be regularly pruned in the same way. A larger greenhouse might solve the problem, but this would make my mistake a rather expensive one! Don’t fret- just like family history research, half the fun of gardening is discovering solutions to thorny problems and I will enjoy solving this one. The obvious lesson here is that unless a plant label states that a tree or plant has been grafted, it is much safer to assume that it hasn’t. The growers’ label on the pot or stem is the best guide to what you actually have in your hands.

The subject of “adopting” tender trees and shrubs has become something of a hot topic (excusing the obvious pun). This year has seen climate change take centre stage. Initiatives to plant more trees have been motivated by the fact that they convert CO2 into oxygen. Although a good long-term initiative, this is rather like me expecting to lose a lot of excess weight through exercise alone, whereas the more successful but tougher approach is to consume less. Leaving it to the trees won’t achieve anywhere near as much in the short run as changing how we behave. And the best trees to plant for any ecosystem ideally need to be native or close natives. Planting banana trees in Essex won’t be as helpful as planting elder, oak, hawthorn, blackthorn and so on.

Climate change (and a good deal of marketing) has encouraged many U.K. gardeners to invest in a wide range of variously described Mediterranean, exotic, tropical or tender trees and plants. There are objections in some quarters to the whole idea of importing non-native stock (along with their pests and diseases) to meet this increased demand. This is a valid point- but global warming is heating our back gardens, we have been importing plants for centuries and we do need to adapt planting for climate change. Personally, I prefer to maintain a reliably frost- hardy low maintenance garden with a few curiosities in containers to provide a bit of bling. Some - the cordylines, hardy palms and phormiums- are fine outdoors all year in our local climate. There are also hardy, heat tolerant shrubs and annuals that require little water or nutrition once established (lavender, for example). Some species have a range of hardy and very tender varieties, such as hedychium (ginger lily), hibiscus and passion flowers; the hardy varieties tend to be more limited in bloom size, colour range and fragrance, but they can provide useful low-maintenance summer glamour.

The degree of hardiness is also a good guide to fussiness. Many exotic trees and shrubs cannot withstand exposed sites, cold winds, poor drainage, shady corners, waterlogging etc. I have thought about the merits of a banana tree- but Peter would consider it pointless to have a banana tree with no edible bananas, he would have to dig up, overwinter and replant it every year, and unless part of a “mini jungle” effort it would look peculiar against the backdrop of ancient deciduous woodland. What the garden designers don’t tell you is that there is a lot of homework to do towards buying, locating and caring for a temptingly wide range of garden exotics. Information on hardiness is always best checked before you buy; RHS hardiness zones H4 -H6 are safe in most of the U.K., while the ratings H1-H3 reflect frost intolerance and often fussier requirements. The RHS ratings are equivalent to USDA hardiness zones 13-6, meaning that the USDA numbers run in the opposite direction and that there is a far wider number (higher and lower) of climate zones beyond our tiny island – see this page if you want to know more about the UK zones.

After centuries of intrepid plant hunting, importing and breeding, many quintessentially British garden plants are “foreign” and regularly imported. It is essential to understand where a tree or shrub originates from, because the growing conditions will have to be replicated. This also means that we can learn from experts in different parts of the world who are caring for the same variety. The best video I’ve watched on cultivating potted lemon trees came from a small grower in California; their knowledge on soil, feed, moisture, light etc. was a masterclass far superior to that of Gardeners World (sorry, Monty!). Equally, I turned to a video produced in Spain to learn more about my fig tree- which is very hardy but needs a lot of sheltered sun to produce ripe figs- and discovered that pruning back a third of the branches to a 6” stub each year in rotation boosted fruit production and overall health no end. This sort of advice doesn’t come on plant labels or sales sites. As with genealogy, looking to the more “distant cousins” in the gardening world can lead to better results and deeper subject knowledge which might one day come in handy elsewhere.

It’s also important to understand what growing conditions are needed both in warmer and colder months. Be prepared to move or protect frost tender plants before the frosts come (in the U.K. this year, that’s a whole month later than in previous years). While containers can undoubtedly help with controlling the soil, feed, moisture and aspect, they will require regular care to maintain optimal health (and fruiting) especially in hot conditions. As the soil in my garden is heavy clay, the drainage can be insufficient for some of the things I want to grow. Raised beds solve this problem for the hardy and annual varieties that need better drainage or different soil, while containers are better for the items that need moving out of frost, snow and cold winds. But all plants in containers are entirely reliant on what you give them. The limiting factors in terms of how many tender plants to grow are the size of my greenhouse and the time I can spend meeting their needs.

Do beware of treating all your frost tender favourites as houseplants during the winter; many of them will be happier resting in an unheated greenhouse or shed with minimal watering, natural daylight and higher humidity than indoors. The shock of a drastic change of environment - too much heat, watering, dry air or incorrect light exposure- can be deadly. While relatives may be impressed to see real lemons on real trees in our warm sitting room at Christmas, the risk of killing them off (the trees, that is) is too great.

At the moment I am clearing my greenhouse of summer veg and giving it a good clean before moving my tender plants in for the winter. All plants and their pots will be cleared of bugs and slugs at the door to minimise infestation. The lemon trees will be placed on gravel filled trays (to ensure adequate humidity) and watered and fed more sparingly throughout the winter. Sappy annual growth of cannas, dahlias, pelargoniums etc is cut to pot level to avoid rot and disease. Scorch and overheating caused by strong winter sun can be avoided with an automatic vent and bubble wrap taped to the window, to diffuse light. The aim is to create a comfortable microclimate for all the plants to get their annual rest.

Good care is all about getting specific. Citrus trees don’t tolerate chlorinated water - the leaves quickly turn yellow and drop. Use rainwater or fill a bucket with tap water and let it stand for a couple of days before using so that the chlorine can evaporate away. One advantage of tap water is that it contains calcium, which all plants including citrus trees need. Some of my methods might seem untraditional, but climate change is also driving a revision of old ways. The canna will be watered minimally; potted dahlias are cut down and left unwatered under the benches (those in my outdoor raised beds are cut down and left in situ; I have never needed to lift, treat and store dahlia tubers). The Birds of Paradise, ginger lilies, aeoniums and other succulents will also rest in the greenhouse with minimal attention. Outside, the Canary Palms, phormiums, cordylines, fig and olive are perfectly happy without any protection – but none are exposed to cold, drying winds or waterlogging. Towards spring, I will check for health and signs of growth, divide, repot, trim etc and start feeding and watering as they each begin to show signs of growth again.

And next summer, the glorious showstopping performance of my exotic specimens - and the figs and lemons, free from air miles, organically grown and picked fresh- will be ample reward for the little extra effort needed to keep them happy.

Indoors, now is just the time to pot up Hippeastrum (“Amaryllis”) bulbs for a champion display at Christmas and throughout the New Year. I soak the bulbs in clean water overnight and pot them up with just enough room for an inch of soil under and around 2/3 of the bulb. Use a heavier pot and insert a tall, strong stake before the stem races upwards. With good light and adequate water (no waterlogging!) they will be more impressive than the blingiest of Christmas decorations- and they make lovely presents too!