December in Zone 6 is the gardener's deep breath. The garden is frozen or nearly so, the average first frost (Oct 15–Nov 1) is a month past, and there is nothing to plant outside. December's work is planning, learning and indoor growing — and it is the month that quietly decides how good next year will be.
What December means in Zone 6
Zone 6 sits in the middle of the American garden map — cold enough for a real winter, with average annual minimums of -10 to 0°F, but with a growing season long enough for tomatoes, peppers and winter squash to finish comfortably. The frost-free season lasts about six months, from mid-April into late October. The average last spring frost falls in the Apr 1–15 range and the average first autumn frost in the Oct 15–Nov 1 range — USDA zone averages, not exact dates, so check your local extension service before planting tender crops outside. Our planting calendar by ZIP turns them into a month-by-month plan.
Zone 6's long winter is the price of its generous summer. With average annual minimum temperatures of -10 to 0°F, nothing tender survives outside, and the useful December garden is indoors: microgreens, herbs, houseplants, seed catalogues and a soil test sent off before spring.
December task list
- Plan next year's garden: beds, rotation, and a seed-starting calendar counted back from the Apr 1–15 last frost. 2. Send a soil sample to your local extension service — winter results arrive in time for spring. 3. Order seeds and supplies while stocks are full. 4. Clean, sharpen and oil any tools missed in November. 5. Check stored produce weekly and remove anything spoiling. 6. Grow microgreens and herbs indoors for fresh flavour all winter.
What to start indoors now
December is the best month to start planning next year's indoor seed schedule. Count back from the average last frost (Apr 1–15): onions and leeks start indoors in late January, peppers and eggplant in February, tomatoes in late February or March. Order seeds now so they arrive in time. Grow microgreens and herbs on the windowsill — parsley, chives and mint in pots, plus radish and pea shoots. Start amaryllis bulbs indoors for winter flowers, and keep houseplants watered sparingly — most grow slowly in winter and need less water than in summer. Use clean pots and fresh seed-starting mix, give seedlings strong light, and keep the soil moist but never waterlogged. Work out exact sowing dates with the seed-starting calculator.
Winter is also the hardest time for houseplants: short days and dry heated air. Move plants closer to windows, keep them away from cold glass and radiators, and water less. Our monstera care guide explains winter care in detail.
What to direct-sow outdoors
Nothing is sown outdoors in December in Zone 6, and no honest guide should pretend otherwise. Indoors, sow microgreens every week for a continuous supply, and sow herbs like parsley (70–90 days to maturity) and chives in pots. If you have a heated greenhouse or a bright, cool sunroom, you can sow lettuce and spinach in containers — but for most Zone 6 gardeners, December sowing means windowsills, not beds. Prepare the bed first — fork it over, remove weeds, rake it level — then sow thinly, water gently, and thin seedlings to the spacing in the table below.
What to transplant now
No transplanting happens in December in Zone 6, indoors or out. The only moving of plants is protective: make sure any potted perennials or tender plants are in their winter spots — a cool garage, a bright windowsill, or a cold frame — and that pots are not sitting in water that will freeze and crack them. Check stored bulbs, onions and garlic for soft spots, and use or compost anything that is spoiling before it affects the rest. Harden seedlings off for a week first, plant on a cloudy day or in the evening, water them in well, and mulch around — but not against — the stems.
What to harvest this month
December harvests come from storage and the windowsill, not the garden. Use stored onions, garlic, potatoes and winter squash — check them regularly for soft spots. Cut fresh herbs from indoor pots, and harvest microgreens as needed. If the ground is not yet frozen solid and you mulched carrots or parsnips heavily, you may still be able to dig a few on a mild day — but do not count on it, and never force frozen ground. Harvest in the morning, when plants are cool and crisp, and pick little and often — regular picking keeps most vegetables producing.
Frost protection in Zone 6
December frost in Zone 6 is deep and settled — the garden is frozen, and frost protection now means insulation and storage, not covers. Check that garlic beds and perennials still have their straw mulch; wind can strip it away, exposing crowns to freeze-thaw cycles. Protect pots from freezing by moving them under cover or grouping them together and wrapping them. And remember the frost dates for planning: the average last frost of Apr 1–15 is the anchor for every spring sowing decision, so keep it in mind as you plan. When a cold night is forecast, water the soil in the afternoon, cover tender plants with frost cloth before sunset, and remove the covers in the morning. The Garden Planner can help you time plantings around your frost dates.
Planting data for December in Zone 6
All figures below are broad published ranges from standard horticultural references, not exact figures — they vary by variety, climate and soil.
| Crop | Action this month | Days to maturity | Notes |
|---|---|---|---|
| Microgreens | Sow indoors weekly | 7–14 | Radish and pea shoots; sunny windowsill |
| Parsley | Grow in pots indoors | 70–90 | Slow to germinate (14–28 days) |
| Onions (stored) | Use from storage | 90–120 | Check weekly; remove soft ones |
| Winter squash (stored) | Use from storage | 90–110 | Keep cool and dry; check for soft spots |
| Garlic (planted) | Dormant under mulch | 180–240 | Do not disturb; check mulch cover |
| Houseplants | Care indoors | — | Water sparingly; move to brightest windows |
Browse the full planting calendar hub for every Zone 6 month, and see the neighbouring months in this zone:
Frequently asked questions
Can I plant anything outside in December in Zone 6?
No — the ground is frozen or nearly so, and nothing will grow. December gardening in Zone 6 is indoors: microgreens, herbs, houseplants and planning for spring.
What can I grow indoors in December in Zone 6?
Microgreens (ready in 1–2 weeks), potted herbs like parsley, chives and mint, and forced bulbs like amaryllis and paperwhites. A sunny windowsill is enough for most of these.
When should I start seeds indoors for next spring in Zone 6?
Count back from the average last frost, Apr 1–15: onions and leeks in late January, peppers and eggplant in February, tomatoes in late February or March. December is for ordering seeds and making the calendar.
How do I care for houseplants in December?
Move them to the brightest windows, keep them away from cold glass and radiators, and water less — most houseplants grow slowly in winter. Check for pests, which thrive in dry heated air.