Top Job for November – Protect Borderline Hardy Plants

Borderline hardy

Borderline hardy simply means that the plant may survive during a mild winter but, in a severe one, may struggle or even not survive.

Hardiness zone

If you’re unsure if your plants are hardy, borderline hardy or tender, the Royal Horticultural Society, (RHS), has devised a hardiness rating system for many plants.

There are several options on how to protect plants, depending on the type of plant and its size.

Mulching plants

Mulching is the ideal way to protect borderline hardy and tender plants, and is essential in colder parts of the country.

It’s super easy and should be done in either autumn or early winter. Autumn for the plants that won’t stand a frost, early winter for those that can take one frost.

Just add between 15cm – 25cm (6-9”) of organic matter, such as leaf mould, bark chippings, straw or sand, around the plants. Do this above for cut back plants like dahlias, cannas and gladioli.  Check regularly over winter and top up where necessary.  Remove in spring before new growth starts to appear.

If you’re in any doubt as to whether or not a plant is borderline hardy then err on the side of caution and add a deep layer of mulch around the base.

Wrapping plants

Sometimes it’s just not practical to lift or move borderline hardy plants.  In that case the best thing to do is wrap them with items that will both insulate from the cold and wind, whilst allowing air to circulate.

Horticultural fleece, straw, hessian and bracken can all be used.

Keep an eye out on the weather forecast and, if very mild spells occur, remove the covers during that time. This helps prevent potential rotting of the plant stems, and replace as soon as temperatures start to dip, or cold winds whip up.

Examples of plants that need to be wrapped are tree ferns and bananas.

When wrapping tree ferns the leaf fronds should be tied in an upright position and then the centre packed out with straw or bracken before wrapping. Then tie the whole tree with a double layer of horticultural fleece.  Add a deep layer of mulch all around the base to help protect the roots from frost.

Mature bananas (Musa basjoo) should have their top growth sawn off, leaving stumps. Then surround the tree with a cage made from canes with chicken wire wrapped around them and secured with cable ties.  Pack out the cage with straw and then cover the cage with hessian or horticultural fleece to keep out the wind.  Covering the top with polythene sheets helps protect the crown from excessive winter wet.

Unwrap plants in May.

Traditionally tuberous plants such as dahlias and cannas are dug up and stored overwinter. But now, in milder parts of the UK, they can be left in the ground after cutting the tops back and covering with a thick mulch.  However, this is a risk, especially if the site is exposed and the soil not particularly well drained.

To be safe, dig up the tubers and store over winter.

Larger specimens of shrubs such as ceanothus, callistemon and pittosporum tobira can be protected by surrounding them with chicken wire. Then pack out the space between the trunk and the wire with straw or bracken.

Smaller specimens can also be protected in a similar way.

The leaves and stems of herbaceous perennials like phygelius and deciduous agapanthus, die down to the ground over winter.  These need to be protected, which can be done by adding a thick, dry layer of mulch on the soil above them.

Evergreen agapanthus planted in the ground should be protected by placing a few layers of horticultural fleece over the leaves.

If you live in a colder part of the country it’s best to grow them in containers and move them into a frost-free conservatory or greenhouse over winter.

Borderline hardy climbers are usually grown on sunny, sheltered walls. But it’s worth adding a deep layer of mulch around the base of the plant in late autumn.

You may be surprised to learn that plants growing in containers can be more susceptible to winter weather than plants growing in the soil!

This is because the roots don’t have the protection of lots of soil around them. Which performs an insulating function, and, as a result, can easily freeze and ultimately die.

If possible, move the pots so that they’re up against a sheltered wall, ideally south-facing, raised above the ground by placing the pot on bricks or pot feet.

This prevents the pot sitting in pools of water as that too can lead to rotting roots.

Then cover all the pot sides with bubble wrap.

Save polystyrene chips from delivery packaging. These can be placed into plastic bags with the tops sealed with gaffer tape to make ‘plant pot duvets’.

You can tie these around the pot.