THE first frosts have turned my nasturtiums into a soggy mess and the herbaceous beds have needed a work over. I tidy up in stages, sparing any survivors until they too collapse – we’ve a long winter ahead, after all. But this is a stark reminder that the tender plants we can’t bring under protection need help to fight off the ravages of the season.

So why does frost kill so many plants? Native species can deal with the cold, but tropicals and sub-tropicals are helpless. When frost occurs, ice crystals begin to form on a plant’s cell walls, causing water from the centre of the cell to penetrate the walls in order to stop damage to the cell.

The plant then becomes dehydrated and leaves start yellowing just like those of dry plants in summer. When the ice crystals melt, the cell can’t absorb this flood of water so it often perishes.

The quicker the switch from warm to cold temperatures, the the greater the damage. You may remember in 2010 stretching out on a deckchair on a fine November day and waking up to a covering of snow the next.

Hardening off young plants, as we do in spring, gradually toughens them. Scientists in India have also just found it helps to regulate gradually the rate at which ice crystals form. They demonstrated one of the benefits of genetic modification by showing this can be achieved by transferring a gene that naturally occurs in some insects and plants to frost-sensitive crops.

Although gardeners can’t use genetic engineering, we can help plants protect themselves. Strawberries put on a fresh flush of foliage after fruiting and, as with hellebores, I leave these sacrificial leaves as winter protection and only clear the foliage away if it collapses and could cause rotting.

I also let neighbours protect each other. After removing straggly geum stems, I leave nearby wallfowers alone over winter and trim away any encroaching stems in spring.

Alternatively, use twiggy stems to shelter vulnerable specimens. Fan-shaped evergreen conifers seem a good choice, but though I’m always in favour of using natural materials, these protections may need to be removed during a mild spell to stop warm, clammy air developing inside.

Building and dismantling these structures every time the weather changes is a hassle and the second edifice is never as good as the first. Plant jackets are better. All you need to do is slip its zip up or down according to the weather.

These jackets come in different sizes and thickness. Choose a cover that’s a little larger than the shrub, otherwise you’ll damage branches when forcing them into an overly tight corset. They’ll rub against the material, suffer through lack of air circulation and may break during heavy snow. One full-proof method is to build a frame round a shrub or small tree, big enough so none of the branches touch the sides when covered with a jacket.

Traditional cloches and covers also work, but flimsy fleece offers little frost protection and steer clear of solid plastic. There’s no air circulation inside and static air loses more heat overnight than the slight movement through fleece.

Mulching provides a protective layer round plants, but only apply it when the ground is soft. The mulch traps heat which rises through the soil. A solid organic mulch may rot crowns, so I use a layer of wood ash or horticultural grit around vulnerable crowns. Remove ash at the end of winter.

Warm air obviously doesn’t rise through pots and compost can become very cold. Group pots together to trap as much warm air as possible and protect the outer layer with bubblewrap or similar.