December and January feel like the dead zone for Pennsylvania gardeners — but they are actually the best months for the work that determines how well your whole season goes. Planning, ordering, starting the very earliest seeds, and taking care of what is already in the ground all happen now, and getting ahead of them makes March and April far less chaotic.
This guide covers everything a PA gardener should actually be doing in December and January — what little can be started indoors, what to check on outdoors, and how to use the quiet months to set yourself up for a productive spring.
❄️ December / January Garden Checklist — Pennsylvania
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What to Start Indoors in December and January
The honest answer for most PA gardeners is: not much in December, and only a few specific crops in late January. Starting too early is one of the most common mistakes — seedlings that outgrow their containers before it is safe to transplant become rootbound and stressed, and they rarely catch up to plants started at the right time. The Old Farmer’s Almanac’s planting calendar tool is useful for calculating back-count dates from your last frost — worth bookmarking before you commit to any start dates.
Late January: Onions and Leeks
Onions and leeks are the legitimate reason to be starting seeds indoors in January in Pennsylvania. Both need 10–12 weeks of indoor growth before they are large enough to transplant, which means a late January start lands them at transplant size in late April — right at the correct outdoor planting window for most of the state.
Onion seeds are slow to germinate and the seedlings look like grass for several weeks, but they are very manageable under lights. Use a quality potting mix, provide 14–16 hours of light per day — a south-facing window in Pennsylvania in January provides maybe 8–9 hours of weak winter light, which is not enough. Give seedlings the 14–16 hours of light they need through PA’s short winter days with an adjustable grow light for seed starting positioned a few inches above the tray, and keep temperatures around 65–70°F.
For our full guides on variety selection and growing, see Growing Herbs in Pennsylvania for any indoor herb work you want to get a head start on as well.
Late January: Celery and Celeriac (Zone 6b–7a)
Celery and celeriac both need an extremely long indoor start — 10–12 weeks minimum — and are suitable for late January starting only in Zone 6b and 7a where the transplant window opens a bit earlier. Celery is notoriously difficult to start from seed and slow-growing; many PA gardeners skip seed starting entirely and purchase transplants in May instead. If you want to try it, start late January in Zone 7a, early February in Zone 6b.
What to Avoid Starting Yet
Tomatoes, peppers, cucumbers, squash, and most warm-season vegetables should not be started in December or January for Pennsylvania. The correct start times for those crops run from late February (peppers) through late March or April (cucumbers, squash). Starting them now produces seedlings that will be severely rootbound and stressed long before it is safe to put them outside.
Keep soil warm and moist while your earliest onion and leek seeds germinate — the right setup pays off across every seed-starting session from January through April. See seed starting kits →
What to Check Outdoors in December and January
Garlic
If you planted hardneck garlic in October or November — the standard fall planting window for PA — it should now be dormant underground. Your primary job is mulch maintenance. Garlic crowns need protection from the severe freeze-thaw cycling that is common in PA winters, particularly in zones 5 and 6. Check that at least 4–6 inches of straw mulch remains over the planting area. If any bare soil has appeared (from wind, settling, or heavy rain), add more straw. For full garlic growing guidance, see our How to Grow Garlic in Pennsylvania guide.
Do not be alarmed if a bit of garlic green is peeking above the mulch in December in Zone 6b or 7a. A small amount of top growth before full dormancy is normal for hardneck varieties in milder zones and does not indicate a problem as long as adequate mulch protection is in place.
Perennial Herbs
Established perennial herbs — thyme, sage, chives, oregano — are dormant and need no intervention. Leave dead stems standing through winter rather than cutting them back now. The dry stems provide some insulation for the crown and support overwintering beneficial insects. Cut them back in early spring when you see new growth emerging from the base.
Rosemary is the exception in colder PA zones. In Zone 5 and 6a, rosemary is marginally hardy at best, and container-grown rosemary should already be inside for the winter. See our Growing Rosemary in Pennsylvania guide for full overwintering detail.
Berry Plantings
Fall-planted strawberries, blueberries, and brambles are dormant and need only a check on mulch coverage. Verify that straw mulch over strawberry beds is still adequate — 3–4 inches over the crowns. Exposed crowns in Zone 5 and 6 are at risk from hard freezes without snow cover.
Planning and Ordering in December and January
This is genuinely the highest-value garden activity you can do in December and January. Seed catalogs begin arriving in December, and the best varieties from specialty seed companies sell out well before spring. If you have specific tomato varieties, unusual peppers, or heritage garlic you want to grow, January ordering is not early — it is just in time.
Penn State Extension’s vegetable production resources for Pennsylvania are worth reviewing during the planning phase — their variety trial data and pest/disease management guides help you make informed decisions about what to grow and how to manage the PA-specific challenges you will face in season.
Use December and January to map your bed rotations on paper. Good crop rotation — moving tomatoes, peppers, and other solanums to a different bed each year; rotating brassicas; following heavy feeders with legumes — is far easier to plan in January when you are not under time pressure than in April when seeds need to go in the ground. Review what grew where last year (keep a simple garden notebook if you do not already) and sketch out the plan for this season.
Order seeds by the end of January at the absolute latest. February ordering still works for most mainstream varieties, but popular heirlooms, unusual peppers, and specific tomato varieties from small seed companies often sell out in January. If you had a variety you loved this year, order it now for next season.
Winter Composting in Pennsylvania
A compost pile does not stop working in winter — it slows dramatically, but it does not stop. Keep adding kitchen scraps through December and January. The pile freezes at the surface but maintains some microbial activity at the core throughout most PA winters, especially in a large pile or enclosed bin.
If your pile is accessible and not frozen solid, turn it once in December and once in January to introduce oxygen and reactivate the decomposition process. Covering the pile with a tarp reduces heat loss and keeps excess moisture out, which helps maintain a better carbon-to-nitrogen balance. By spring, the material added over winter will have started breaking down and will be ready to incorporate into beds. For a full breakdown of composting best practices in PA, see our Complete Composting Guide for Pennsylvania Gardeners.
Zone Notes for December and January
| PA Zone | December Action Items | January Action Items |
|---|---|---|
| Zone 5a/5b | Ensure maximum mulch on garlic and strawberries; bring any borderline perennials inside; order seeds | Begin seed catalog ordering; onion/leek start late January; check overwintering container plants in garage |
| Zone 6a/6b | Same as above; check rosemary if left outside under protection; review compost pile | Start onions and leeks late January; plan bed rotations; order all seeds by end of month |
| Zone 7a | December can bring mild stretches — garlic may show green; do not remove mulch. Check overwintered container herbs | Start onions and leeks mid-January; celery start late January; garlic green above mulch is normal — leave mulch in place |
Do not remove mulch from garlic during December or January warm spells. Pennsylvania regularly gets several days of 50–60°F weather in January, and it is tempting to pull back mulch when you see green garlic tops. Resist. The mulch is protecting the crown from the hard freeze that is almost certainly coming in the next few weeks. Leave it in place until late March or early April when the risk of temperatures below 20°F has passed.
Frequently Asked Questions
Can I start any seeds outdoors in December or January in Pennsylvania?
Not meaningfully for most crops. The ground is frozen or near-frozen, and soil temperatures in most of PA are well below the germination threshold for any vegetable. Cold-stratification of seeds (some tree seeds, certain perennials) can be done outdoors in winter, but vegetable seed starting for the coming season is entirely an indoor activity in December and January.
When should I start tomato seeds in Pennsylvania?
Late February through mid-March for most of PA, depending on your zone and last frost date. Tomatoes need 6–8 weeks of indoor growing time before transplanting after your last frost. Starting in December or January produces plants that will outgrow their pots and become rootbound before it is safe to transplant them outside.
What do I do with my herb garden in winter in Pennsylvania?
Hardy perennial herbs (thyme, sage, chives, oregano) can stay in the ground with no intervention — just leave the dead stems standing through winter for protection and cut them back in spring. Rosemary is marginal in Zone 5 and 6a and does best overwintered indoors or with heavy protection. Tender herbs like basil are annuals and are done for the season — compost them. See our Growing Herbs in Pennsylvania guide for full detail by herb type.
Is it too late to plant garlic in December?
In most of PA, yes — garlic planted in December after the ground has hardened will not have time to establish roots before hard freezes hit. The optimal PA window for fall garlic planting is October through early November, when soil temperatures are still above 50°F but air temperatures are cooling. Zone 7a gardeners can sometimes push into late November. If you missed the fall window, wait until spring and plant as early as the ground can be worked — spring-planted garlic produces smaller bulbs but is still worthwhile.
Related Guides: See our What to Plant in November in Pennsylvania guide for fall wrap-up tasks, our How to Grow Garlic in Pennsylvania guide for winter garlic care detail, and our Pennsylvania Soil Guide for planning soil amendments to make before spring planting begins.
