Fleece, hessian, bubble wrap or straw — which winter protection should I use?

Winter protection fails when the material is right and the target is wrong — bubble wrap round a shrub, straw against a crown that then rots, polythene straight onto leaves. Deciding what you are protecting first makes the material choice obvious.

Step by step

1. Decide what is actually at risk.Leaves and soft shoots, the crown at soil level, the roots, or the whole plant? These need different materials and getting this wrong is the usual cause of failure.
2. For a plant in the ground with tender top growth, use fleece on a frame.Held off the foliage, sealed at ground level. Add a second layer for a harder frost rather than switching material.
3. For a herbaceous crown, use a dry mulch as a collar.Heaped over and around the crown, wider than the crown, with a hollow left at the stem so water drains away from the neck rather than sitting against it.
4. For a pot, insulate the pot, not the plant.Wrap bubble wrap or hessian round the outside, raise the pot onto feet so it drains, and move it against a wall out of the wind. A root ball frozen solid for days is what kills a container plant.
5. For a tender trunk or growing point, build a cage and fill it.Chicken wire or mesh around the plant, packed loosely with straw or bracken, with hessian over the outside to shed some rain. Loose packing traps more air than tight packing.
6. Add drainage wherever winter wet is the real threat.For Mediterranean and silver-leaved plants, a grit collar and improved drainage does more than any wrapping. Insulating a plant that is drowning does not help it.

Tips

• Trapped air is the insulator in every one of these materials. Anything packed tight, or anything soaked, insulates far less well than the same material dry and loose.
• Whatever you use has to be secured against wind. More winter protection is lost to a gale in December than is defeated by cold in January.
• Leave a way to check underneath. Protection you cannot inspect hides rot, mice and the moment when the plant starts growing through it.
• Second-hand and improvised materials work: cardboard boxes over a crown for a hard spell, an upturned bucket over a small plant, old net curtains as fleece for one night.

Common mistakes

• Wrapping a whole plant in bubble wrap — It cannot breathe, condensation forms inside it, and where the plastic touches leaves it conducts heat away rather than insulating. Bubble wrap is for the outside of pots.
• Packing straw hard against a stem or crown — Wet packed straw holds water against the neck of the plant all winter. Rot at the collar kills more borderline plants in Britain than frost does.
• Protecting the leaves of a container plant and ignoring the pot — A pot has soil exposed to cold air on all sides and freezes right through in a way that garden soil does not. The root ball is the vulnerable part.

Alternatives

• Horticultural fleece: Breathable, light-passing, sheds some rain. The default for protecting foliage and whole small plants. Must be held off the leaves and sealed at ground level. Buys a degree or two per layer; degrades in sunlight over a season or two.
• Hessian: Breathable, tough and long-lasting, but passes little light. Best for wrapping trunks, tree fern crowns and the outside of pots, or as an outer layer over straw. Sags badly when soaked, so it needs supporting.
• Bubble wrap: Excellent insulation for the outside of a pot, where the enemy is a frozen root ball rather than frosted leaves. Non-breathable, so it must never go over foliage, and it should not seal the top of a pot or the compost cannot breathe or drain.
• Straw, bracken or dry leaves: Cheap bulk insulation for crowns and root zones. Traps a great deal of air. Must be kept dry to work and must not be packed against a stem, or it holds water there and causes rot. Usually needs netting or a mesh cage to hold it in place.
• Bark or composted mulch: The practical everyday crown protection. Easy to apply, stays in place, does not need caging, doubles as soil improvement. Less insulating than straw for the same depth.
• Coarse grit: Not insulation at all — drainage. A grit collar around a crown and grit worked into the planting hole is what keeps a silver-leaved or Mediterranean plant alive through a wet winter, which is a bigger risk here than cold.
• Polythene sheet: Only ever as a rain shield above a plant, held clear on a frame with the sides open. Wrapped round a plant it conducts heat away where it touches, traps condensation and cooks the plant in morning sun.
• A cold frame, cloche or unheated greenhouse: Structural protection for small plants and pots that you can move. Gives a bigger and more reliable margin than any wrapping, and lets you check and water without dismantling anything.

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