How do I protect a plant from frost when it is too big to cover?

Beyond a certain size, whole-plant protection stops being possible and the question changes from 'how do I keep it undamaged' to 'how do I make sure it survives and regrows'. For most large tender plants those are very different targets, and the second one is usually achievable.

Safety

What you'll need

• Thick mulch — bark, straw, bracken or composted material
• Hessian or fleece (optional)
• Mesh or chicken wire and stakes
• Soft twine

Step by step

1. Identify the part that has to survive.For a tree fern it is the crown at the top of the trunk, where next year's fronds come from. For a banana it is the base and the underground rhizome. For a grafted tree it is the graft union. For most tender shrubs it is the base and the root system. Protecting that is the whole job.
2. Mulch the root zone heavily.A thick layer over the whole root spread, kept clear of the stem itself. This is the highest-value single action for almost any large plant, because a root system that survives will regenerate top growth even if the top is lost.
3. Wrap or pack the growing point if there is one you can reach.A tree fern crown is stuffed loosely with straw and covered so rain does not sit in it. A banana is cut back and the base caged with mesh and packed with dry material. Loose packing traps more air than tight packing and is less likely to rot.
4. Clear the ground beneath the canopy on frost nights.Bare, firm, moist soil holds the day's heat and radiates it upward at night. Grass, deep mulch and leaf litter insulate the ground so that heat stays down where the plant cannot use it. In the frost-risk period, pull mulch back from under a fruit tree in blossom.
5. Use the structure around it.A large plant against a house wall, in the lee of a building or under a high canopy is already several degrees better off than one in the open, and moving a large plant is sometimes more realistic than protecting it every winter.
6. Reduce the sail area if wind is part of the problem.A large tender plant that is rocked all winter suffers root damage and a wet, opened root socket regardless of temperature. Shortening the top growth or securing it reduces both.
7. Accept the losses above head height and plan the recovery.Wait until spring growth shows what survived before cutting anything. Many large tender plants regrow strongly from an undamaged base and look better for it by midsummer.

Tips

• Wrapping a trunk protects the trunk, not the top. For a plant whose growing points are all at the top, trunk wrapping is largely decorative unless the crown is protected too.
• A temporary frame — four stakes and a wrap of hessian, open at the top — around a large shrub cuts wind chill and desiccation substantially without needing to enclose it.
• Group large pots together against a wall for winter. The mass of the group buffers the temperature swing and each pot shelters its neighbours.
• If the same plant needs an elaborate rescue every year, it is in the wrong place or is the wrong plant for the site, and there is no shame in concluding that.

Common mistakes

• Wrapping a large plant tightly in polythene — It conducts heat away where it touches, traps condensation against the plant all winter and causes rot. Rot in a wrapped plant is a far commoner outcome than cold damage would have been.
• Leaving wrapping on into spring — Wrapped growing points break into growth in the dark, produce pale weak shoots and then get scorched when it comes off. Take protection off in stages once growth starts.
• Protecting the top and ignoring the roots — Roots are what regenerate the plant. A protected top over a frozen or drowned root system is a plant that dies anyway.

Questions people ask