There is a widespread misconception, even in RHS handbooks, that frost tolerant cacti don’t exist. In fact it is more the wet and humidity that they cannot handle, in a dry environment there are many that can handle frost; indeed many Agaves are very cold tolerant if not wet. Thankfully we do not get a combination of wet and cold, especially if the plants are planted in hillocks of grit and sand, which is what we do with the slightly less tough specimens.
Although these plants, especially when mature, are highly adapted to water shortages, they will grow much faster if given regular summer water and a little low nitrogen food. Growth will then be really quite quick; Agave americana for example can reach 2m in 6 years from a small off shoot.
Many of these plants are extremely spectacular, architectural leaves and amazing forms, stunning and huge flowers which are produced regularly. They are a core part of the drought tolerant garden and come in sizes from around cup sized to 10m and more tall.
I have grown these plants since I was a teenager and had a frost tolerant collection of over 100 cacti from the genuses Lobivia and Rebutia. They were housed bone dry in an unheated outdoor greenhouse all winter in the UK down to -10 centigrade and used to flower extravagantly every summer and then one winter day my aunt placed a wet towel in the greenhouse. 2 weeks later the whole lot died from frost; a demonstration indeed of the need to keep these plants dry over the winter. It was not the frost alone that killed them but it with the humidity in the air from the towel.
Highly recommended plants are shown in blue.
Echeveria x Black Knight
Nolina microcarpa (Bear grass)
Sedum rupestre ‘Angelina’
Semperviums (House leeks)
Delospermum cooperii, Delospermum lineare and Malephora crocea
These stunning succulents (well the flowers at least – magenta pink, bright yellow and orange/red) are often recommended for this climate zone. However after way too many failures with them I cannot endorse these recommendations. Water them to establish them in summer and they are munched on by hordes of slugs and snails until they are twigs, don’t water them and they don’t establish enough to overwinter. Of course they can be overwintered indoors and then planted out in late spring….but life is too short.