December gardening jobs

SuttonsHere are your gardening jobs for the month – with added links if I have more information on the subject. They are all still relevant to do now if you’ve fallen behind or if the weather’s been bad.

Week 1

The deeper into winter we get, the more the lines of ‘what to do when’ become fuzzy. Please check the previous weeks’ 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.

Species tulip T. tarda
Species tulip T. tarda doesn’t continually need replacing, as it naturalises well

Winter can be a tough time for birds in terms of water and food, 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.

Continue to plant 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.

Holly Silver Queen
Holly Silver Queen (actually a male)

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.

Ornamental grasses and bamboos can be cut back and tidied up.

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.

Large tubs that are at risk of cracking in the frost should be covered 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. Roses are generally shallow rooted and can become loose in the soil.

Tree and shrub seeds and berries can still be harvested and sown, once ripe.

Pelargoniums (geraniums) can be cut back, de-leaved, watered less, not fed, and kept relatively dormant and just moist, over the winter. Re-potting, watering, and feeding in the spring will bring them back into active growth. They must be kept at about 10°C (50°F) while dormant.


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.

Cornus Midwinter Fire
Cornus Midwinter Fire, silver birch and me at RHS Harlow Carr’s Winter Walk

Tidy up leaves from around borders. They can be added to the compost heap, or placed 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. Leaf mould is an excellent soil improver, and can also be used as a seed-sowing medium.

Watch out for downy mildew and black spot on winter pansies.

Check chrysanthemums regularly for signs of white rust.

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 is still uncommon but 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. Mealybugs and scale insect nymphs are commonly found and should be picked off. Grape vines 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 carefully for any pests and diseases they may have picked up in the garden. Unhappy looking plants can always be tipped 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.

Dead heads of globe artichokes
Dead heads of globe artichokes provide winter structure

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 tap roots, such as dandelions and mallow, from newly cultivated areas. Clay soils, in particular, can be more workable in autumn, as they are no longer baked hard, but not yet sodden and sticky with winter wet. Mulching will help to improve the soil structure.

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.

Lily of the valley (Convallaria) can be dug up, potted and forced in the greenhouse at this time of year. Rhizomes that are 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.

Root cuttings can be taken now and throughout the winter. Papaver (perennial poppies), Verbascum (mullein) and Phlox are suitable examples.

Dig new flower beds as the weather allows. Don’t work on them when it’s very wet, as walking on sodden soil can cause compaction.

Look out for crown rot and brown rots (Sclerotinia) on died down perennials, especially if you are on a clay or poorly drained soil. Be aware that many diseases will overwinter in the soil or on plant debris. Antirrhinum rust and Delphinium black blotch, as well as Sclerotinia, will lay dormant and re-infect plants when they come up the following year. It may be necessary to replant new specimens in another place if the problem is severe.

Digging the soil, especially bare patches or newly cultivated land, 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 and washing away of valuable nutrients. 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 wind break is enough where there is no natural shelter. Straw, bracken, or something similar can be used to pack around deciduous plants and protect them from frost. A wooden frame with clear polythene stretched over it can do a similar job without blocking light from evergreens, but don’t let the polythene touch the foliage, as condensation at these points could freeze, or cause rot.

Pruning and renovation of many deciduous trees, shrubs, and hedges can be carried out from now throughout the dormant season. It is easier to see what you are doing when the branches have no leaves. Suitable examples are Fagus and Corylus. Exceptions are tender plants, and also Prunus species (e.g. ornamental cherries, plums, and almonds), as these are vulnerable to silver leaf if pruned in the autumn or winter. Evergreens are best left until the spring.

If there is snow in your area, then you may need to brush this 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.

Cyclamen can be sown now, with a bottom heat of 12-15°C (54-60°F). They will need about 14 months to produce flowers.

Maintaining some air movement in the greenhouse or conservatory, even when the weather is cold, will help to reduce problems with fungal diseases such as Botrytis (fluffy grey mould). Opening vents slightly (but not enough to create damaging draughts) in the morning, and then closing them in the early afternoon to conserve heat, will offer enough ventilation to help keep fungal problems at bay.

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.

Now is a good time to consider installing garden lighting, water pipes, and drainage, and to make plans for garden projects.