How do I mulch a crown to protect a plant over winter?

The crown is the point where shoots meet roots, and on a herbaceous plant it is the only part that has to survive the winter. Protecting it is easy, and doing it slightly wrong is the commonest way to lose a plant to rot in a winter that never got particularly cold.

Safety

What you'll need

• Coarse mulch — bark, composted bark, straw, bracken or leaf mould
• Coarse grit or sharp sand (optional)
• Gloves
• Mesh or netting if using straw on a windy site

Step by step

1. Clear the area around the crown first.Take off soggy dead foliage, fallen leaves and any old mulch that has broken down into a wet mat. Mulching on top of a rotting layer traps moisture exactly where you do not want it.
2. Improve the drainage underneath if the soil is heavy.Fork coarse grit into the top of the soil around the crown, or plant on a low mound in the first place. On clay this is worth more than any amount of mulch, because winter wet kills more borderline plants here than cold does.
3. Choose a coarse, open material.Bark, composted bark, straw, bracken or coarse leaf mould. What you want is a material that holds air and does not compact into a wet blanket. Fine compost and grass clippings do the opposite.
4. Heap it over and beyond the crown.Roughly the depth of a hand, and spread wider than the crown so cold cannot get in from the side. Deeper is better for a genuinely tender plant, provided it stays open and dry.
5. Leave a hollow at the neck.This is the step that matters. Keep the mulch off the stem or the centre of the crown itself so air moves there and water drains away rather than being held against it. A collar, not a plug.
6. Hold it in place if it will blow.Straw and bracken need netting, a ring of mesh or a few pegged branches on an exposed site. Bark generally stays put.
7. Check it after heavy rain and after wind.Mulch settles, washes aside and packs down. A quick look every few weeks catches a collar that has slumped into a plug or blown away entirely.
8. Take it off when growth starts.As soon as shoots begin pushing up through it, pull it back off the crown and spread it around the plant instead. Leaving it on shades and rots the new growth.

Tips

• The insulation is the trapped air, not the material. Loose, dry and coarse works; wet, packed and fine does not, whatever it is made of.
• A ring of grit immediately around the crown, under the mulch, gives fast surface drainage exactly where rot starts.
• For plants that resent any wet on the crown at all — many silver-leaved and alpine plants — a pane of glass or clear plastic propped above on bricks, open on all sides, is the correct protection rather than mulch.
• Mice and voles nest in a deep straw collar over winter and will chew the crown. If that has happened before, use bark rather than straw.
• Mark the position of anything herbaceous before you mulch it. In spring you will otherwise dig into a plant you have covered up.

Common mistakes

• Packing mulch hard against the stem — It holds water permanently at the neck of the plant. Rot at the collar, not frost, is what kills most plants that were mulched for winter.
• Using fine compost or grass clippings — Both compact into an airless wet layer. That has no insulating value and excludes air from the crown as well as holding water on it.
• Mulching a crown that is sitting in waterlogged ground — You are insulating a drowning plant. Fix the drainage or accept that the plant is in the wrong place; mulch on top of standing water makes it worse.
• Leaving the collar on into late spring — New shoots pushing through a deep collar come up etiolated and rot at the base, and the mulch keeps the soil cold well past the point where the plant wants to grow.

Questions people ask