December garden tasks: Pacific Northwest

This article contains affiliate links. As an Amazon Associate, we earn from qualifying purchases, at no extra cost to you. See our full disclosure.

Cold snaps, east winds and snow

Most west-side cold damage happens during short arctic outbreaks. Dry east winds through the Gorge and the Fraser Valley push temperatures well below normal and dry out foliage. OSU Extension's guide to protecting landscapes in freezing weather recommends:

• Watering dry plants before a freeze, since moist soil holds more heat.
• Covering tender shrubs and vegetables with cloth or row cover that reaches the ground.
• Moving containers to shelter or grouping them against the house.
• Gently brushing heavy, wet snow off shrubs and small trees before branches break. Leave ice-coated branches alone, because knocking the ice off breaks them.

Wait to prune freeze-damaged shrubs until spring growth shows which parts are alive.

Plants under eaves and overhangs miss most of the winter rain. Check their soil before a freeze and water if it is dry. The same goes for containers on covered porches. Marginally hardy plants such as hebes, some hardy fuchsias, rosemary, phormium and young citrus or olives benefit most from a temporary cover or a sheltered wall during the coldest nights.

Winter harvest

West-side gardens can keep producing through winter. Per WSU Extension, plants make little new growth in December, so you are harvesting from a standing crop. OSU Extension's December calendar lists the month's harvest and protection jobs.

• Kale, collards and Brussels sprouts often taste sweeter after frost. Pick lower leaves first.
• Leeks, parsnips and carrots store in the ground. A layer of straw or leaves makes it easier to dig them during cold spells.
• Keep row cover on lettuce, spinach and other greens to protect them from frost and pounding rain.
• Check garlic and overwintering onions. Pull weeds while they are small, and top up the mulch if heavy rain has thinned it.

Harvest during thaws, not while plants are frozen. Frozen leaves bruise and turn mushy when handled, but the same plants are usually fine once they thaw. Overwintering cauliflower and purple sprouting broccoli won't produce until late winter or spring. Just keep them protected and free of slugs until then.

Holiday trees and plants

For a cut tree, plain water is all you need. Oregon State University research found that additives (including vodka, sugar and commercial mixes) do nothing to help a tree keep its needles. Make a fresh cut across the trunk, set the tree in a stand that holds plenty of water, and keep it full.

WSU Extension's guide to holiday plants covers poinsettias, Christmas cactus, amaryllis and other gift plants. Give them bright light, keep them away from drafts and heat vents, and let excess water drain. If you are cutting greenery from your own conifers and hollies, take it from the back or interior of the plant, and use pruning cuts that leave a side branch. OSU Extension's guide to harvesting boughs explains how to do it sustainably.

Living (potted) Christmas trees: keep them indoors for as short a time as possible, ideally about a week, in the coolest room you have, and keep the root ball moist. Move them back outdoors gradually to a sheltered spot. Plant them out during a mild, workable spell, and allow for the full mature size of the species. Many sold as holiday trees grow into large forest conifers.

Dormant pruning: what to do now and what to wait for

• Now: remove broken, dead or storm-damaged branches whenever you find them.
• Wait until late winter: apple and pear pruning. OSU Extension recommends pruning apple trees in late winter for stronger growth and better fruit. The March guide sets early March as the practical deadline.
• Wait for dry weather: cherries, plums and other stone fruit. OSU's bacterial canker guide explains why rain-season wounds are a problem.
• Wait for February: roses. Pruning too early invites dieback when cold weather follows (see the February guide).

Trees, shrubs and evergreens in winter

• Storm checks: after windstorms, look for split leaders, hanging limbs and heaving roots on young trees. Cut broken branches back cleanly to a side branch or the trunk collar. Leave large or high work near power lines to a certified arborist.
• Stakes and ties: loosen ties on trees planted last year so they don't girdle the trunk. Remove stakes from trees that have been in for a full growing season and stand on their own.
• Planting: container trees and shrubs can still go in during mild, workable spells. Don't plant into frozen or waterlogged ground.
• Rhododendrons and other broadleaf evergreens: leaves that roll up and droop in hard cold are a normal way of reducing water loss. They flatten again when it warms. Water container-grown evergreens during dry east-wind spells, since frozen or dry potting mix leaves them unable to replace moisture.

Lawns, moss and slugs

Stay off frozen or waterlogged lawns. Footprints on frosted grass leave dead patches, and traffic on saturated soil compacts it. Clear leaves off the turf after storms. Moss is at its most visible this time of year, so use December to map where it grows. OSU Extension points to shade and poor drainage as the usual causes.

Slugs stay active in mild, wet spells. Keep protecting overwintering greens and young garlic, as described in the September guide. Lift boards or pots near beds on mild days and remove the slugs sheltering underneath.

Lawns need nothing else this month. Don't fertilize, and don't apply moss products to frozen turf. If the mower comes out on a dry day, set it high and keep the blade sharp.

Plan next year's garden

• Write down what grew well and what didn't, while the season is fresh in your memory. Note slug hot spots and beds that flooded.
• Plan a crop rotation so tomatoes, potatoes and brassicas aren't in the same beds year after year.
• Order seed catalogs. West-side gardeners will start onions and early brassicas indoors in January and February (see the January guide).
• Look over stored winter squash, potatoes and dahlia tubers. Throw out anything soft or moldy.
• Sketch where water stood after the big storms. Those spots are candidates for raised beds or a rain garden next year.
• Clean, sharpen and oil pruners, loppers and saws now. The main dormant-pruning season for apples, pears and blueberries starts in January.

East of the Cascades

East of the Cascades, December is fully winter (zones 5–7). Snow cover insulates perennials, so let it stay on beds, but brush heavy snow off upright evergreens. During dry, open spells when the soil isn't frozen, water evergreens and recently planted trees. University of Idaho Extension notes that winter drought adds to cold injury. Check that winter mulch is still in place over strawberries, garlic and tender perennials, and add more if wind has blown it off.

Use de-icing salt sparingly near beds and lawns, and don't pile salty snow from driveways onto planting areas. Look for rodent damage at the base of young fruit trees when the snow melts. Mice and voles feed on bark under snow cover, so pull mulch a few inches back from trunks and use hardware-cloth guards where damage has happened before.

Common mistakes

Frequently asked

Do Christmas tree preservatives work?

Oregon State University research found that additives don't help. A fresh cut and a stand kept full of plain water is what keeps a tree fresh.

Can I prune my fruit trees in December in the Pacific Northwest?

You can remove dead or broken branches at any time. For apples and pears, OSU recommends late winter for the main pruning. Prune cherries and plums in dry weather to limit bacterial canker.

What vegetables can I harvest in December in Seattle or Portland?

Kale, collards, leeks, Brussels sprouts, chard, parsnips, carrots and hardy greens under cover can all be picked in December. They are harvested from a standing crop, because almost nothing grows in December's short days.

Sources

1. December Garden Calendar (EM 9684), OSU Extension
2. Protect landscapes when freezing weather moves in, OSU Extension
3. Treat your Christmas tree to vodka? Only if you want to waste it, Oregon State University
4. Caring for Your Holiday Plants, WSU Extension
5. Harvesting evergreen boughs from your woodland, OSU Extension
6. Prune apple trees in late winter for stronger growth and better fruit, OSU Extension
7. Fall and Winter Vegetable Gardens for Western Washington, WSU Extension Snohomish County

Read next