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Getting tender plants through the winter

Monday, 8 November 2010  |  Admin

After the last winter's long cold snowy spell we are all probably (and very justifiably) concerned that we may have to deal with another spell of cold weather in 2019. However we have to be prepared for a cold snap at some stage during February, and possibly into March, as was the case last year. Many parts of the country have already experienced frost or snow and most of the really tender tropical plants should have been moved indoors by now or protected outdoors.

You only have to follow a plant related group on Facebook to see how inventive people are when it comes to keeping precious plants over the winter, the attic, kitchen, nothing is out of bounds to the lover of frost-tender plants. Having stowed as many plants indoors, how many of us will admit to getting up at 3 am and going outside in the freezing cold with a torch to check the greenhouse heater, or to tuck an extra piece of fleece over a plant?

Looking around our house I'm sometimes reminded of the line from a Flanders and Swann song:

'The garden's full of furniture ...the house is full of plants!'.

Some points to ponder..

When it comes to nurturing frost-tender plants, there are some factors which can make the difference between success and failure:

Get to know the micro-climates in your garden - favoured spots for over wintering plants are the base of a south-facing wall, but anywhere that will afford some protection against rain and snow will help. My tender Agapanthus survived the winter 2017/18 in a dry area under a thick evergreen hedge complete with an overhang of Ivy.

Provide extra protection with several layers of fleece or netting. This is preferable to bubble film as it allows the plants to breath during warmer spells.

Plants which are kept fairly dry survive better than wet plants – the more concentrated the sap the lower the temperature at which it freezes. Feed plants with a high potash, such as tomato, food, rather than one high in nitrogen which promotes lush leafy frost-prone growth.

The age of plant will have a bearing on its chance of survival - take Mandevilla laxa for instance, a beautiful climber with scented flowers, known to be somewhat tender. The plant we had at our old nursery site (pictured right) got to be 10 years old, and survived a cold winter down to minus 10°C. It died back to within a few inches of the ground, but new growth rapidly appeared in spring and it flowered profusely that summer. A younger plant would most likely have succumbed. Sadly, we had to leave it behind when we moved our nursery. 

Insurance is the best policy: if possible where you have more than one plant of a particular variety, spread these around - we tend to do this a lot at the nursery to make sure we have stock from which to propagate. This also gives us the opportunity to find out the conditions in which a plant may or may not survive. 

A long spell of minus 1, especially if this lasts during the day, can do more damage than a short sharp moderately severe frost. Protracted temperatures only just below freezing will gradually permeate down through the soil and into outbuildings such as sheds and garages, which may however protect against that short, sharp frost.

To illustrate this, Charlie, our mad Springer Spaniel, who is unfortunately no longer with us, carefully attached a piece of a Selenicereus (a tropical cactus which I'd always assumed to be very tender) to his fur. He then went outside to roll in the snow, where it became detached. The temperature that night dropped to minus 10 or lower, so I was rather surprised when the rock solid cutting I found the next day, after being gently thawed, didn't turn to mush but started to grow! 

At our latitude in the UK winters can be long and cold, but they would be much colder if it weren't for the gulf stream, which keeps us much warmer than other parts of the world at the same latitude. Here in Suffolk, at a latitude 52 degrees north, we are classed as being in RHS hardiness zone H4 which means we can expect at worst lows of around minus 9 – 10 degrees C. We were in N. Carolina a few years ago, where in theory the temperatures can be lower, BUT due to much the lower latitude (just 35 degrees north of the equator) the winters are much shorter, so low temperatures don't last as long. Someone we met over there was very concerned about her Camellia flowers, her comment was something along the lines of 'We get minus 15 (degrees C.) in January' (here I nodded sympathetically, with only a slight touch of schadenfreude).... 'but, gee, by February it's back in the low 20's !!' , whereupon I felt more than a slight pang of jealousy.

Heating of greenhouses 

Strangely, the temperature in an unheated greenhouse can fall lower than that outside on a clear frosty night.  I won't bore you with the reasons, but be aware that such a structure probably won't keep a prolonged, or sharp, frost out without the help of a heater. Nor will a garage or shed protect against such a frost.  Once you start heating, insulation in the form of bubble film or fleece is imperative if you want to keep your heating bills from sky-rocketing. Bubble film is great but does produce a lot of condensation which can drip on the plants. We find a couple of layers of fleece are effective, with less of a problem with drips. 

An unheated greenhouse does at least provide protection from winter wet, so will be suitable for plants which will take some frost if on the dry side. 

Survive or thrive?

Sometimes a plant will have appeared to survive the winter then fades away when it attempts to grow away in the spring. Even if a plant which is tricky to over winter does survive, perhaps we should question if it is really worth growing if it’s not going to grow, flower or fruit sufficiently to fulfil our expectations.

Learn to interpret the forecast,

I have noticed on several occasions the forecasters tell us cheerfully that ‘it’s going to be a cloudy night which will keep frost at bay’, only for the skies to clear, the wind to drop with the result that the temperatures plummet. I find the ‘Weather Radar’ app on my phone, which shows temperature and cloud cover, very useful I agree you can’t do much (other than the 3 am dash around the garden with a pile of fleece) as you watch the clouds disappear and imagine your precious plants freezing to death, but it does give you an idea of the significance of cloud cover.

Do plants feel ‘wind chill’?

Be aware that many forecasts use ‘feels like’ as well as the actual temperature, the lower of the two being the wind chill. This really only applies to humans (and other animals). However evergreen plants can be severely affected by cold drying winds when their roots are frozen, leading to desiccation of leaves and possible death of the plants. The remedy here is to protect any newly planted evergreen shrubs or trees with a few layers of netting to baffle the wind.

Cold is good!:

Some of us might dream about global warming nudging our winters towards being frost free so that we can grow Bananas and the like outdoors year round. While this would be nice, give a thought to the following points.

Pests and diseases could become problematic if we lose winter frosts – not only will the check on the local bug population be reduced, but there’s also the risk that bugs from warmer climes will take up residence with possibly disastrous results.

We should also remember that many plants require a cold winter to become fully dormant so they can perform their normal growth cycle. This applies to many important food crops - apples, for instance, have a 'minimum chill requirement', meaning that without a certain length of time below a certain temperature they may fail to initiate flowers and fruit for the following summer:

Many seeds need frost in order to break their dormancy so they can germinate, for example Primulas, and many other hardy plants.

Aesthetically, frost, snow and ice adds another dimension to gardens, picking out the detail of leaves and flowers that we might not normally notice. Go to Instagram, for example, and search for frosted leaves, frosted foliage to bring up some amazing images. And when spring comes along, maybe we appreciate it all the more.

Whether or not the climate is going to change in ways to significantly affect us as gardeners in our lifetime remains to be seen. Without drifting too much into the subject of climate change, as I understand it, the difference in temperature of the air masses either side of the jet stream is reducing as the polar air mass warms rapidly. This means that the jet stream is meandering more, forcing the weather to get stuck in certain, possibly more extreme, weather patterns and for more prolonged periods. Recent experience seems to support this, so we should probably be prepared to adapt our gardens to reflect these changes.