When the cold arrives and sometimes it's just a little bit too cold for some plants. Often it is the combination of wet and severe cold that is damaging. This is usually worse in poorly drained heavy clay soils.
The joy of English gardens is the cosmopolitan variety of plants we grow, Japonicas from Japan, Orientalis from Asia. We grow many New Zealand plants, lots from a Mediterranean regions and so forth. It is not surprising some cope better with extremes than others and I find it helpful to know something of where plants originate.
Easy Things You Can Do To Help Your PlantsIt is easy to get a broom and knock the snow from the leaves of large leafy evergreens. Lots of snow on the leaves can cause cell damage appearing like scorch on even frost hardy plants. Morning sun from the east can cause more damage when the frozen leaves are thawed too quickly and increase the chance of cell damage. Snow can also weigh down the branches and cause them to break.
Plants in pots will be colder than the equivalent plants buried in the ground. Stand them somewhere a little more sheltered from the weather for a few days or even pop some fleece on top.
Many plants find minus ten nights uncomfortable. I prefer to be a little more cautious as I do have rather a lot of plants to worry about!
These heavy cold spells seem to perpetually affect the same plants and as it warms up in March and the growing season begins the extent of damage will only then become apparent.
Plants That Struggle with Extremes of ColdBay plants will struggle if not already well established with a strong root system under the ground and a little shelter from the house. I have a sixteen year old bay bush on the south west of the house which has taken a few knocks over these cold winters but it's been mature enough to survive.
Rosemary will hate the cold and young plants in wet positions will probably not survive. If they are in a drier soil and there's not too much to freeze they may make it. Those in pots will be colder than those planted in the ground as the cold air can penetrate the pot. I have moved mine into the shelter of the eaves of my house and I hope this is enough. None of the silvery Mediterranean herbs will love this weather, this includes sages and lavenders, these don't like to be over wet or sit in water. I will be doing my best to see that these drain when the volumes of slushy snow thaws into the container. If they begin to thaw then refreeze it's cruel.
New Zealand pants can struggle with the real lows. Phormiums are quiet often knocked back to the ground. If they are old plants the can appear dead but will re shoot and completely recover. Pittosporums and the contorted corokia can struggle. I think this is why they are best planted close to a warm wall. Watch the snow on the massive dinner plate leaves of the fatsias and knock it off if you can.
Choisya ternata always takes a hit from the cold . Often the juvenile shoots turn brown but usually they make a full recovery. When you consider they have originated from Mexico, Arizona way, I think they do pretty well to deal with our climate.
Agapanthus are bulbs that really dislike wet soggy cold conditions and when you remember they come largely from South Africa should we be in any way surprised! I've broght my agapanthus under the eves of the house for protection from the wet and the cold and it makes a real difference especially as the sheltered pots don't get so wet. My dahlias have survived the winter here too.
Alpine plants are usually low growing, often good ground cover and great in cold but desperate for a well drained position resembling the alpine slope they used to call home. On the slopes lots of water passes through and drains away. Slopes often cover with snow but when a thaw occurred water would seep away through the rocky hillside. Once plants are forced to sit in unhappily soggy conditions, planted in a flat border or a heavy soil or both, for any length of time they become susceptible to fugal diseases which are free to breed and usually rot the roots.
Californian plants speak for themselves. I use to have a prized wonderful ceanothus for about 7 years and suddenly half of it died, I blame the snow of 2009. When it tried to grow in the spring half the plant had turned brittle and brown. It may have been susceptible to botryosphaeria a fungal disease due to the cold soggy conditions or possibly its roots could no longer support its full size due to freezing cells in the freezing wet clay soil. The combination of extreme cold combined with extreme wet conditions is quiet a cruel challenge.
The beloved Garry Elliptica also has its origins in California and I have three well established plants thriving and they have coped well with the cold winters. I think the significant point is that they were well established before they had to face the cold. Two of mine are against walls and one is amongst shrubs, none are standing alone and exposed.
I would always plant warmer climate plants in the spring rather than autumn and give them a chance to establish before cold sets in and they do prefer to be sheltered by a warm wall or a thick shrubbery or fence. If you can improve the drainage with grit in heavy soils it would be beneficial except when we have drought like in 2011 and plants want all the moisture they can get! How will we gardeners ever get it right!