Here are your December gardening jobs with links if I have more information on the subject. Check November’s tasks to see if any are still relevant to do if you’ve fallen behind or if the weather’s been bad. For the ideas behind how and when we do gardening tasks, visit the Gardening Jobs page. See November’s gardening tasks here. Read about my garden in December here.
Week 1
The deeper into winter we get, the more the lines of ‘what to do when’ become fuzzy. Please check the previous week’s gardening jobs – there still may be tasks you can carry out.
Alpines can be sown from seed, as they need a period of cold to break their dormancy. A moveable cold frame can be positioned over the sown area to protect it from excessive wet. Alternatively, the seeds can be put in the fridge, for sowing next spring. This will really upset your family.
One of the main December gardening jobs is to keep the birds alive, so keep supplies well topped up. Once you start feeding, don’t stop – they will come to depend on it.
Pot up Hippeastrum (Amaryllis) bulbs, and bring them back into active growth with regular watering and feeding. They should give you beautiful flowers in the new year.
Plant tulip bulbs at the latest this month. Some species tulips go on year to year, some are treated as bedding, and replaced every year.
Apply a mulch to protect plants that are borderline hardy. The plants’ own leaves, e.g. Kniphofia, can be tied up and used as protection for the crowns underneath.
Helleborus niger (Christmas rose) blooms can look unsightly when splashed with muddy raindrops. A mulch will reduce this, and cloches can always be used where practical.
Ensure all standpipes and irrigation lines are drained, to avoid damage caused by water freezing inside them.
Plan for the future – one of the lovely December gardening jobs is planting roses. Avoid planting in areas where roses were previously grown otherwise new introductions may suffer from replant diseases (rose sickness).
Shrubs normally pruned hard in the spring – such as Buddleja davidii, Cornus alba and Lavatera – can be cut back by half now, to prevent wind rock and neaten their appearance.
Ensure any pruning of Acer (maple) and Betula (birch) is completed before the end of the month to avoid bleeding of sap from cuts.
Make sure you have removed all shading from the greenhouse panes, to maximise light levels. If applying insulation, attach it only to the sides and north-facing roofs to let in as much light as possible.
Avoid buying Poinsettia that have been accidentally chilled, particularly those sold from street stalls on cold days. Once home, place them in a warm, light place, away from drafts, to make sure they last for as long as possible.
Place hyacinths in a cool, bright place in the home. If it’s too warm, the leaves will elongate and the flowers will fade quickly.
Week 2
If you’ve already got your Christmas tree up, prevent needle drop by choosing a pine (Pinus) or fir (Abies) tree instead of a Norway spruce (Picea abies). Don’t place the tree near fires or radiators. Cut trees last longer in a stand with a reservoir. Cut off the bottom 5-7.5cm (2-3in) of the trunk to allow the tree to drink.
To save holly berries from the birds, for use in Christmas decorations, net a few branches, but leave the rest for the wildlife.
Take hardwood cuttings of ornamental shrubs such as Berberis, Buddleja, Salix, Forsythia, Ligustrum, and Rubus. Some deciduous climbers, such as honeysuckle, can also be propagated in this way.
Leave dead Penstemon stems as they are until the spring, as they will help to protect the crowns from cold. Mulching over crowns in colder areas will also help.
Clean and sharpen secateurs ready for pruning deciduous trees and shrubs over the winter. Special ceramic tools are available to allow awkwardly shaped and angled blades to be sharpened with ease. Spare springs and replacement blades can also be purchased for more expensive models.
Cut back and tidy up ornamental grasses and bamboos.
Many clay soils will now be unworkable until spring. Mulching will help to improve and maintain soil structure.
Protect alpines from the wet, if you have not done so already.
Cover large tubs at risk of cracking in the frost with bubble wrap, hessian or fleece, to insulate them over the winter.
Clean out water butts if not yet done, or install extra ones.
Lightly prune bush roses now, if not done already, as reducing their height will prevent wind-rock. Shallow-rooted roses can become loose in the soil.
Harvest and sow tree and shrub seeds and berries once ripe.
Cut back Pelargoniums (geraniums) now. Re-potting, watering and feeding in the spring will bring them back into active growth. Keep them at about 10°C (50°F) while dormant. Watch out for grey mould.
Week 3
Take note of the most colourful dogwoods (Cornus), Salix and white-stemmed Rubus shrubs when visiting gardens open to the public, or in garden centres, and consider planting them yourself, for a winter display.
Tidy up leaves from around borders. Add them to the compost heap, or place in separate bins to make leaf mould. Some leaves, such as plane and sycamore, are slow to break down and can delay you using your compost if you mix them into the general heap. Use leaf mould as a soil conditioner and a seed-sowing medium.
Watch out for downy mildew and black spot on winter pansies.
This is also a good time to transplant trees and shrubs growing in unsuitable positions. However, if they are more than a couple of years old, you are unlikely to be able to remove an intact enough root ball to ensure the plant’s survival in its new position, and you may be best advised to leave well alone.
Check tree stakes and ties are secure and will withstand the winter weather; make sure that ties are not strangling trunks or branches – they may need loosening.
Holly leaf blight can be spread in wet weather.
Many pests can overwinter in nooks and crannies in the glasshouse structure and in the bark of woody houseplants and vines. Pick off mealybugs and scale insect nymphs. Grapevines often have old bark stripped off before winter, to cut the number of hiding places.
When bringing plants into the house or greenhouse, check them for any pests and diseases. Tip unhappy-looking plants out of the pot to look at their root balls for signs of over or under-watering, or for soil pests like vine weevil larvae.
Put up insulating material such as bubble wrap on the inside of the greenhouse, if not already done.
In mild weather, weeds will still appear. Hoe regularly to keep them in check.
Week 4
Raise patio containers on to feet or bricks to avoid them sitting in the winter wet.
Remove stakes and other supports as the last late-flowering herbaceous plants die down for the winter.
Now can be a good time to dig up perennial weeds with long taproots, such as dandelions and mallow, from newly cultivated areas. Mulching will help to improve the soil structure.
One of the main December gardening jobs is planning for next season. Order seed catalogues for next year’s bedding and perennials, if not already done. It is not too late to research and order summer-flowering bulbs for planting in the spring or during the winter.
Michaelmas daisy mites on Aster novi-belgii cultivars can be a problem. Other asters, such as Aster novae-angliae cultivars and Aster ericoides cultivars, have more resistance.
Grey mould or Botrytis can be problematic in wet weather.
You can still order and plant container trees and shrubs, and large semi-mature specimens for planting later in the winter when bare-root plants are no longer available.
Force Lily of the valley (Convallaria) in the greenhouse. Rhizomes kept in a frost-free greenhouse over winter will stay in active growth, but given a little heat, they will be ready to flower shortly after replanting in the garden next spring.
Heat and/or insulation will be needed to keep the greenhouse frost-free. A fan or paraffin heater should do the trick in small glasshouses. Maintaining higher temperatures will need more careful planning, and a better greenhouse heating system. Greenhouse insulation can help keep out the frost from the whole, or from a section, of the greenhouse.
Take root cuttings now and throughout the winter. Papaver (perennial poppies), Verbascum (mullein) and Phlox are suitable.
Dig new flower beds as the weather allows. Don’t work in wet weather, as walking on sodden soil can cause compaction.
Look out for crown rot and brown rots (sclerotinia) on perennials, especially if you are on clay or poorly drained soil. Antirrhinum rust and Delphinium black blotch will also lie dormant. It may be necessary to replant new specimens in another place if the problem is severe.
Digging the soil will expose pest larvae and eggs to birds and frosts, as well as clearing weeds and improving soil structure. Don’t leave soil uncovered for too long, however, as it runs the risk of erosion. Black polythene sheeting will protect it instead of planting or mulch.
Protect newly planted trees, hedges, and shrubs from wind and cold. A temporary netting windbreak is enough where there is no natural shelter. Use straw, bracken, or something similar to pack around deciduous plants and protect them from frost. A wooden frame with clear polythene stretched over it can do a similar job with evergreens, but don’t let the polythene touch the foliage, as condensation at these points could freeze, or cause rot.
One of the main December gardening jobs is the pruning and renovation of many deciduous trees, shrubs, and hedges. It is easier to see what you are doing when the branches have no leaves. Suitable examples are Fagus and Corylus. Don’t prune tender plants, Prunus species (ornamental cherries, plums, and almonds), or evergreens.
If there is snow, brush it off the branches of conifers. Heavy snowfall can splay branches and spoil the shape of the tree.
Phytophthora root rots can cause dieback on mature trees and shrubs. Wet winter weather and poorly drained soils are likely to encourage this problem on susceptible woody plants.
Rabbits, deer, and squirrels can be a nuisance as the weather gets colder, gnawing the bark from shrubs and trees. Placing guards around new woody plants is advisable.
Sow Cyclamen with a bottom heat of 12-15°C (54-60°F). They will need about 14 months to produce flowers.
Maintain some air movement in the greenhouse or conservatory, even when the weather is cold, to reduce fungal diseases such as Botrytis (grey mould). Opening vents slightly in the morning, and then closing them in the early afternoon, will offer enough ventilation.
This is a good time to clean all your old pots and seed trays so that they are ready for next spring’s flurry of activity. Thorough cleaning will cut pest and disease problems and will make your propagation and sowing yield much greater.
Consider installing garden lighting, water pipes, and drainage, and make plans for garden projects.
To see January’s gardening jobs, visit my week-by-week guide here.
December gardening jobs updated June 2022