Assuming the weather is open, you can: Plant roses, fruit trees and bushes, deciduous shrubs, trees and hedges. Also Jerusalem artichokes in the vegetable garden. Prune apples and pears, wall climbers and shrubs, soft fruits, outdoor vines.
Do not prune stone fruits (cherries, plums, etc.) because of the risk of infection with silver leaf disease.
Spray dormant fruit trees with tar oil to control overwintering larvae and aphids.
Move shrubs around the garden if you are doing a bit of re-design work.
Check stakes and ties, especially after high winds, to make sure they are firm and not chafing the plants.
Construct and remake paths, walls and other structures during non-frosty periods. Check paths and other walkways for slippery algae, mosses and lichens and if necessary treat with moss killer.
Dig any undug empty parts of the garden, adding manure or compost if you have it, and lime, but leave a month between applications of each if you are applying both lime and manure or compost.
Fork over lightly any compacted soil between ornamental plants. Do not dig deeply or you will damage the roots.
Protect the crowns of slightly tender herbaceous plants with bark or leaf mould, and shrubs with a screen of polythene or netting on the windward side. Check plants, especially trees and roses, for holes in the soil around the base — this is caused by wind-rock and if left, they will fill with water and the roots will rot. Frost will also loosen the soil around plants, which should be gently firmed down.
Turf new lawns in suitable weather. January can often be a mild month and established lawns may grow quite long. Lightly trim with a sharp mower, but only in open weather when no frost or cold winds are forecast for some time. If you are not sure, it is better to leave well alone.
Force rhubarb by covering the crowns with old buckets or other light-excluding material.
In snowy weather, shake the snow off evergreens, especially conifers, as it can pull the branches down and even break them, permanently spoiling the shape.
Make a hole in the ice on fishponds by placing a tin can of hot water on the surface for a few minutes.
Harvest winter vegetable crops. Cover winter brassicas with fine mesh against bud damage.