December garden tasks: Northeast and Long Island

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Finish winter mulching

December is when the ground usually freezes in zones 6–7 (it often froze in late November in zone 5). That is the signal to finish winter mulching. The goal, as covered in the November guide, is to keep frozen soil frozen through winter thaws so perennials, strawberries and late-planted bulbs are not heaved out of the ground.

• Spread 2–4 inches of straw, shredded leaves or evergreen boughs over perennial crowns, the garlic bed and fall-planted perennials. Boughs from the Christmas tree work well after the holidays.
• Mound soil or compost over the base of grafted roses in zones 5–6 if you have not done it yet.
• Leave mulch pulled back a few inches from tree and shrub trunks so voles do not nest against the bark.

On Long Island and along the coast, the ground may freeze and thaw several times instead of staying frozen. Mulching is still worthwhile because it evens out those swings. Our guide to winter protection for perennials covers which plants benefit most.

Evergreens, snow and ice

Evergreens keep losing water through their foliage all winter. Per UMass Extension, plants that went into winter well watered hold up best. If December is dry and the ground has not frozen yet, one more deep watering is worthwhile.

After a snowstorm:

• Brush heavy, wet snow off evergreens and multi-stem shrubs with an upward sweep of a broom, working from the bottom up. Iowa State Extension recommends removing snow gently before it freezes.
• Leave ice-coated branches alone. Knocking ice off breaks the branches. Let it melt naturally.
• Have a certified arborist handle large broken limbs, especially near power lines. Small broken branches can be cut back cleanly to a side branch or the trunk collar at any time of year.

Keep salt and salty snow off beds, as planned in November. UMass Extension recommends using as little deicer as possible and putting sand or grit down for traction near planting beds.

Keep the cut Christmas tree fresh

A freshly cut tree that never goes without water stays green and holds its needles through the holidays. Per Penn State Extension, which works with Pennsylvania's large Christmas tree industry:

• Choose a tree with flexible needles that do not drop when you run a branch through your hand.
• Cut a thin disk off the base of the trunk just before putting it in the stand. The cut seals with sap within hours, which blocks water uptake.
• Use a stand that holds plenty of water. A fresh tree can drink a lot in the first days, and if the water level falls below the base, the cut seals and you have to recut.
• Use plain water. Additives are not needed.
• Keep the tree away from heat registers, fireplaces and space heaters.

After the holidays, cut off the branches and lay them over perennial beds as winter mulch. Many towns also chip trees for mulch.

Holiday plants

Poinsettias: per Penn State's Year-Round Gardening series, give them bright, indirect light, keep them away from cold drafts and hot air vents, and water when the surface of the soil feels dry. Poke a hole in or remove decorative foil sleeves so water can drain; sitting in water is the most common way to kill one. Protect the plant from cold on the trip home, because even brief exposure to freezing air damages the leaves.

Amaryllis: plant the bulb with the top third above the soil line, water sparingly until growth starts, and give it bright light. Turn the pot every few days so the stalk grows straight. After it blooms, cut off the flower stalk and keep the leaves growing in a sunny window. That foliage builds the bulb for next year. Illinois Extension has a good overview of after-holiday care for gift plants.

Christmas cactus: water when the top inch of soil is dry and keep it out of drafts. Bud drop usually means the plant was moved or its temperature changed suddenly.

Pruning: what you can and cannot do now

The best time to prune most deciduous trees, shrubs and fruit trees in the Northeast is late winter, roughly late February through March, as covered in the March guide. December is too early for most pruning. Cuts made before the coldest weather can lead to dieback around the wound, and you cannot yet see what winter will damage.

Fine to do in December:

• Remove dead, broken or storm-damaged branches.
• Cut evergreen boughs for decorating from holly, fir, pine, boxwood and juniper. Take them from the back or interior of the plant and cut back to a side branch so the shape stays natural.
• Prune oaks during the dormant season. The New York State DEC asks residents not to prune oaks from April through July, when beetles that spread oak wilt are active. Oak wilt has been found in parts of New York, including Long Island.

Do not prune now: spring bloomers such as forsythia, lilac, rhododendron and azalea, which already carry their flower buds, and bigleaf hydrangeas. Per UMass Extension, prune spring-flowering shrubs right after they bloom. See how to prune flowering shrubs.

Force bulbs for late-winter color

Leftover spring bulbs make good winter houseplants. Tulips, daffodils, hyacinths and crocus need a cold period before they bloom. Per Michigan State University Extension, pot them now and keep them cold (above freezing but below about 45°F) for roughly 12–15 weeks in an unheated garage, a cold basement or the refrigerator, away from ripening fruit. Then bring them into a cool, bright room. Bulbs potted in December bloom in March or April.

• Use a pot with drainage holes and set the bulbs shoulder to shoulder with their tips just at the soil surface.
• Water them after potting and keep the soil lightly moist during the cold period.
• Paperwhites and amaryllis do not need a cold period. Paperwhites usually bloom a few weeks after planting.

Planning and the shed

• Keep a garden record. While this year is still fresh, write down which varieties did well, which failed, where pests hit, and what got planted where. Crop rotation for tomatoes, squash and brassicas depends on remembering last year's layout.
• Read your soil test results. If you sent a sample in October, the report tells you whether to lime and how much. Lime can go on the vegetable garden any time the soil is not frozen or saturated.
• Check stored tender bulbs and produce. Look over dahlias, cannas, squash, onions and potatoes in storage and remove anything soft or rotting before it spreads.
• Feed the birds, and leave the seed heads standing. The coneflower and black-eyed Susan stalks left up in fall are doing their job now.

Common mistakes

Frequently asked

Can I prune my fruit trees in December?

It is better to wait. Northeast Extension guidance favors late winter, from late February through March, for apples and pears, and prunes stone fruits such as peaches even later, close to bloom. Pruning in early winter can lead to cold injury around the cuts. Remove broken branches now and save structural pruning for February and March.

Is it too late to plant bulbs in December?

If the ground can still be dug, plant them. Bulbs kept indoors over winter usually dry out or rot. On Long Island the soil often stays workable into December. Water them in and mulch.

How do I keep a poinsettia for next year?

Keep it in bright light and water it normally through winter. Cut it back in spring and grow it outdoors or in a sunny window over summer. To rebloom, it needs long, uninterrupted nights (complete darkness) for several weeks starting in early fall. Most people treat it as a seasonal plant, which is fine.

Should I water my garden in December?

Only if the ground has not frozen and the late fall was dry. Then a deep soak for evergreens and plants put in this year is worthwhile. Once the soil is frozen, stop watering.

Sources

1. Caring for Your Cut Christmas Tree, Penn State Extension
2. Poinsettia Care, Penn State / WPSU Year-Round Gardening
3. Protecting Evergreens in Winter Q&A, UMass Extension
4. How to Prevent Ice and Snow Damage on Trees and Shrubs, Iowa State University Extension
5. Pruning Shrubs, UMass Extension
6. Protect Your Oaks, New York State Department of Environmental Conservation
7. Gift of Growth: Nurturing Your Holiday Plants, University of Illinois Extension
8. Forcing Spring Bulbs to Bloom Indoors, Michigan State University Extension

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