December in the garden: prune, force, protect

Daniel Platteau

In short: December is the darkest month of the year, but not an empty one. It is the time to prune apple and pear trees (not plums or cherries), to force your own chicory in the cellar, to keep a hole in the pond ice without smashing it, to let the hedgehog sleep and to store tools and batteries away from the frost. And a few shrubs flower and fill the air with scent in the middle of winter.

The leaves have fallen, the borders are resting and the days have never been shorter. A gardener could put the spade away until March. That would be a pity: with no leaves, you can finally see the framework of the trees, which makes pruning easier; the cellar can produce a fresh vegetable; and the garden still needs a few jobs done to get through the winter.

After September, October and November, here are the jobs that really matter in December.

December in Belgium: the darkest month

The Royal Meteorological Institute (RMI) puts it plainly: December is the first month of meteorological winter and the darkest of the year. At Uccle there are on average barely 45 hours of sunshine in the whole month. The daily mean temperature there drops from about 5 °C at the start of the month to about 3 °C at the end.

For the garden, a few RMI figures sum up the month at Uccle:

So snow remains rare in the lowlands, and in some years none falls at all. Since 1901, Uccle has had only eleven white Christmases. But winter can bite: the December cold record at Uccle is −17.3 °C, on 30 December 1950, and the thermometer has dropped below −20 °C in the Ardennes, at Rochefort, Vlessart (Léglise) and Wirtzfeld (Büllingen). The Ardennes also get the most rain, up to about 170 mm in the month.

In practice: make the most of dry, frost-free days to prune and plant, and leave the soil alone when it is frozen or waterlogged.

Pruning apple and pear trees

This is the big winter job in the orchard. The Royal Horticultural Society (RHS) advises pruning apple and pear trees while they are dormant, between leaf fall and bud burst, usually from November to early March. With no leaves, the framework is easy to read.

The aim: an open-centred, goblet-shaped crown with well-spaced main branches. Light and air can circulate, which improves fruit quality and limits disease. The RHS method, in order:

1. Remove dead, diseased or damaged wood, including any parts affected by canker.
2. Remove crossing branches, keeping the better-placed one.
3. Take out vigorous shoots growing into the centre, and low branches that get in the way.
4. Shorten the previous year's growth by a quarter to a third at the tips of the main branches, just above an outward-facing bud.
5. Leave the young side shoots: they are the ones that will form fruit buds.

The golden rule: don't overdo it. The RHS advises removing only about 10 to 20% of the canopy each year. Beyond that, the tree responds by sending up watershoots, vigorous upright shoots that bear no fruit. An old, neglected apple tree is brought back over several winters, not one.

Trained fruit trees (cordons, espaliers) are pruned mainly in summer; in winter, simply thin out overcrowded spurs.

Maples, birches and vines: prune before the sap rises

Some trees "bleed" heavily if pruned in spring: sap runs out of the cuts. The December advice from the RHS shared by the Met Office, the UK weather service, notes that once the leaves have fallen you can prune deciduous trees, hedges and shrubs with less risk of sap bleeding in spring. Japanese maples and grape vines are best pruned now. The same principle applies to birches.

Bare-root planting: still possible, frost permitting

December extends the bare-root season that opened in November: trees, shrubs, hedges and roses can go in as long as the soil is neither frozen nor waterlogged. The full method is in November in the garden. If a parcel arrives during a frost, heel the plants in, roots covered with soil, until the weather turns milder.

Snow, ice and ponds: the right reflexes

Snow on shrubs. A thick layer of wet snow can bend and break branches, especially on flat-topped hedges, narrow conifers and evergreens. BBC Gardeners' World magazine advises knocking it off with a long-handled broom, not forgetting the leading shoots that give the shrub its shape. Go gently: in hard frost, frozen twigs are brittle.

The pond. The RHS recommends not letting a fish pond freeze over completely, as this can be fatal to the fish. A floating ball, a small air pump or a pond heater keep an opening. If ice has formed, hold a saucepan of hot water on the surface until it melts a hole. Never crack the ice with a hammer: the RHS points out that this is harmful to fish.

Pots. Move pots left outside close to a sheltered wall and raise them on feet so water can drain away: a waterlogged rootball that freezes suffers far more than one that is merely cold. Protection measures are covered in detail in our guide to overwintering garden plants.

Let the hedgehog sleep

The hedgehog hibernates as soon as the temperature drops below 10 °C, explains Natagora, the Belgian nature conservation association. It builds a nest of moss and dead leaves sheltered by a woodpile, a hedge or a shelter. It wakes from time to time and then draws on its fat reserves, which can be fatal, especially for young animals in their first winter. It wakes for good around April.

What this means for the garden in December:

• don't disturb piles of leaves, wood or compost without checking what they shelter, and don't burn them;
• leave an untidy corner: a hedge, a pile of branches, dead leaves under the shrubs;
• make a gap of about 10 cm in fences so it can move between gardens;
• if you find a weakened hedgehog, contact the nearest wildlife rescue centre (CREAVES or CROH in Wallonia) rather than caring for it yourself.

What flowers in December

A winter garden need not be empty. Several hardy shrubs flower in the depths of winter, and many are scented. All of these are rated at least H5 by the RHS, which means hardy in Belgium in most situations.

Add to this list the Christmas rose (Helleborus niger), to which we devote a full article, and the viburnums, presented one by one in our article on viburnums. A cut sprig of winter viburnum in a vase scents a whole room.

In the vegetable garden: forcing your own chicory

Belgian endive, known as chicon or witloof in Belgium, is a Brussels invention, and December is the right time to grow it at home. Its story is told in the Brussels-Capital Region's inventory of intangible cultural heritage, where "the cultivation and forcing of chicory in Brussels" has been listed since 2021. The hypothesis it considers most plausible credits its development, around 1840, to François Bréziers, head gardener of the Botanical Garden at the Porte de Schaerbeek in Saint-Josse-ten-Noode. Production then spread to the "chicory triangle" of Brussels, Mechelen and Leuven, and Brussels chicory was being exported from 1872.

The principle: a witloof chicory root, grown all summer, is deprived of light in winter. It then produces a second shoot, white, tender and less bitter: the chicon. At home, the RHS method is simple:

1. Lift the roots of witloof chicory (in November or December); discard any with a crown less than 2.5 cm across.
2. Cut back the foliage to 2.5 cm above the crown. Roots waiting their turn keep well packed horizontally in sand, somewhere cool.
3. Pot up a few at a time, upright, in a large pot of moist compost, crown level with the surface: about five roots to a 25 cm pot.
4. Put them in complete darkness, under an upturned bucket or black plastic, at 10 to 15 °C: a cellar is ideal.
5. Harvest after about four weeks, when the chicon is around 15 cm tall, by cutting it at the base. A second, smaller chicon may grow back.

By starting a new pot every two to three weeks, you can harvest until spring. No roots in the garden this year? Make a note for May: witloof is sown in spring to be forced the following winter.

Indoors: Christmas plants and lack of light

December is the month of poinsettias, Christmas cacti, holly and mistletoe. Before believing everything said about them (the deadly poinsettia, holly that must have prickly leaves…), read our 15 Christmas plant myths checked. An amaryllis planted in early November should flower for the holidays.

With 45 hours of sunshine in the month, light is the main problem for houseplants. Move them closer to windows: winter light is less harsh than summer light. One precaution shared by the Met Office: at night, cold air builds up between the window and the curtain, which can damage tender plants on the windowsill. For plants that really lack light, see our guide to grow lights. And cut back on watering: most plants grow little in winter.

Putting away tools and batteries

December is the ideal month for maintenance: cleaning and sharpening secateurs and shears, oiling blades, repairing fences, trellises and the garden shed. The Met Office sums it up: this work "will pay off generously" in spring.

The lithium batteries of cordless tools need special attention. Ryobi advises storing them somewhere dry, ideally between 5 and 20 °C, away from radiators as well as extreme cold. An uninsulated garage that freezes is therefore not a good place: bring them into a cupboard in the house. Don't store them completely empty or fully charged for months, check their charge every two to three months in winter, and clean them before putting them away, away from metal objects (nails, screws) that could cause a short circuit.

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December at a glance

Sources

BBC Gardeners' World. (2018, 21 December). Brush snow from plants. PressReader. https://www.pressreader.com/uk/gardeners-world/20181221/284240941492876

Brussels-Capital Region. (2021). La culture et le forçage du chicon à Bruxelles [Inventory of intangible cultural heritage]. patrimoine.brussels. https://patrimoine.brussels/liens/pci/chicon

Met Office. (n.d.). 5 gardening tips for December. https://weather.metoffice.gov.uk/warnings-and-advice/seasonal-advice/your-home/gardening/gardening-tips-for-december

Natagora. (2021). Accueillir le hérisson [Natural garden fact sheet, updated 11/2021]. Réseau Nature. https://reseaunature.natagora.be/fileadmin/Projet_Reseau_Nature/Fiches_JNat/JNat_accueillir-herisson.pdf

Royal Horticultural Society. (n.d.-a). 10 award-winning scented plants for winter. https://www.rhs.org.uk/plants/articles/misc/winter-flowering-shrubs-for-fragrance

Royal Horticultural Society. (n.d.-b). 10 award-winning winter-flowering shrubs. https://www.rhs.org.uk/plants/trials-awards/using-award-winning-plants/10-award-winning-winter-flowering-shrubs

Royal Horticultural Society. (n.d.-c). Apples and pears: winter pruning. https://www.rhs.org.uk/fruit/apples/winter-pruning

Royal Horticultural Society. (n.d.-d). How to grow chicory. https://www.rhs.org.uk/vegetables/chicory

Royal Horticultural Society. (n.d.-e). Ponds: December. https://www.rhs.org.uk/advice/in-month/december/ponds

Royal Horticultural Society. (n.d.-f). Silver leaf. https://www.rhs.org.uk/disease/silver-leaf

Royal Meteorological Institute of Belgium. (n.d.). Climat de la Belgique mois après mois : décembre. https://www.meteo.be/fr/climat/climat-de-la-belgique/mois-apres-mois/decembre

Ryobi. (n.d.). How to store garden and power tool batteries. https://uk.ryobitools.eu/get-inspired/how-to-store-garden-and-power-tool-batteries/