What You Can Plant in January Zone 6a Pennsylvania

January in Zone 6a Pennsylvania is cold, often grey, and the ground is solidly frozen. Average temperatures sit between 20°F and 35°F (-6°C to 2°C), and snow is common.

To most people, the garden looks completely dormant.

But experienced Pennsylvania gardeners know that January is actually one of the most productive months of the year — not outside, but indoors, at a desk with a seed catalogue, or under grow lights in the basement. Get January right, and your entire growing season benefits.

This guide covers everything you should be doing right now in Zone 6a Pennsylvania — what to start indoors, what to direct sow under cover, what to plan, and what to leave firmly alone until the weather warms.

Understanding Zone 6a in Pennsylvania

Zone 6a covers a large portion of Pennsylvania, including areas around Pittsburgh, Allentown, and much of the state interior. In Zone 6a, your average minimum winter temperature falls between -10°F and -5°F (-23°C to -21°C).

For planting purposes, the key dates to work backwards from are:

• Average last spring frost: April 15–30 (use April 22 as your planning date)
• Average first fall frost: October 10–20
• Growing season length: approximately 160–175 days

Every indoor seed-starting date in January is calculated from that April 22 last frost date. Count the weeks backwards and you will know exactly when each crop needs to be sown.

Seeds to Start Indoors in January

January is the month for the slowest-growing crops — the ones that need a very long indoor head start before they can go outside. These are not beginners' crops, but they are worth every bit of the effort.

Onions

Onions are the number one crop to start in January in Zone 6a. They need 10–12 weeks indoors before transplanting, which puts your start date squarely in the last two weeks of January.

Sow seeds thickly in a shallow tray filled with fine seed-starting mix, about ¼ inch apart. Cover lightly, mist, and place under grow lights at 65–70°F. Germination takes 7–10 days.

Once seedlings reach 3–4 inches tall, trim them back to 2 inches to encourage thicker stems — a technique called "tipping" that most growers skip but makes a significant difference.

Recommended varieties for Pennsylvania: Walla Walla, Candy, Ailsa Craig, Copra (excellent keeper).

Leeks

Leeks are even slower than onions and benefit from an early January start. They need 10–12 weeks to reach transplant size and prefer to go into the garden in late March or early April under a cloche or row cover.

Sow into deep trays or cells, barely covering seeds with mix. Germination is slow — up to 14–21 days at 60–70°F. Be patient. Once up, keep them trimmed to 3 inches to build strong roots. Leeks are cold-hardy enough to transplant while frosts are still possible.

Good varieties: King Richard (fast-maturing, great for baby leeks), Lancelot (bolt-resistant), Lincoln.

Celery and Celeriac

Celery is notoriously difficult to start from seed — it needs light to germinate (do not bury seeds), consistent moisture, and a long 10–12 week indoor period. But if you can pull it off, homegrown celery has a depth of flavour that supermarket celery never achieves.

Sow on the surface of moist seed-starting mix, press gently, and do not cover. Maintain a temperature of 70–75°F. Germination can take 2–3 weeks. Start celery in the first two weeks of January to give it the full time it needs.

Celeriac (celery root) follows the same timing and is arguably easier to grow than celery. It stores brilliantly through winter once harvested.

Perennial Herbs

Slow-growing perennial herbs like rosemary, lavender, and thyme can be started from seed in January, but be aware that germination is inconsistent and slow — sometimes 3–4 weeks. Start these early to allow plenty of time before the transplanting window.

• Rosemary: Needs warmth (70°F+) to germinate. Very slow. Start by January 15.
• Lavender: Benefits from cold stratification before sowing. Surface sow, keep moist.
• Thyme: More reliable from seed. Surface sow under lights.

Slow-Growing Flowers

If you want snapdragons, pansies, violas, or petunias blooming early in spring, January is the right time to start them. These need 12–14 weeks indoors, putting them at transplant size right as Pennsylvania's last frost clears in late April.

What to Do Outside in January Zone 6a

Very little direct sowing is possible outdoors in Zone 6a Pennsylvania in January — the ground is frozen and temperatures are too low for seed germination. However, there are still productive outdoor tasks.

Cold Frame Crops

If you set up a cold frame or low tunnel in autumn and planted cold-hardy crops then, January is the time to harvest them. Overwintered crops that can survive a Pennsylvania January in a cold frame include:

• Mâche (corn salad) — extremely cold-hardy, can survive temperatures well below freezing
• Spinach — overwintered plants often survive Zone 6a winters under cover
• Kale — already in the ground will continue to produce slowly in mild spells
• Claytonia (miner's lettuce) — surprisingly cold-tolerant under cover
• Overwintered scallions

Do not try to sow new seeds into a cold frame in January in Zone 6a — soil temperatures are too low for germination. Wait until late February or early March for that.

Garden Planning

January is the single best month to plan your entire growing season. Time spent planning in January pays dividends every single week from April through October. If you have not already done it:

• Draw your bed layout and decide what goes where this year
• Practice proper crop rotation — move brassicas, nightshades, and alliums to new beds
• Calculate how many of each plant you need based on bed space
• Build your seed-starting schedule working backwards from April 22
• Order seeds from your preferred suppliers before popular varieties sell out

January Gardening Tasks Checklist — Zone 6a Pennsylvania

• ✓ Order seeds for the full season before varieties sell out (do this in the first week of January)
• ✓ Clean and sanitise seed trays and pots from last season with a dilute bleach solution
• ✓ Set up or check grow lights — replace bulbs that are more than two seasons old
• ✓ Buy or mix fresh seed-starting medium — do not reuse last year's potting mix for seeds
• ✓ Sow onions and leeks in the last two weeks of January
• ✓ Sow celery and celeriac in the first two weeks of January
• ✓ Cold stratify seeds that need it (lavender, some perennial flowers) in the refrigerator
• ✓ Check stored vegetables — remove any that are rotting before they spread
• ✓ Check overwintering bulbs (dahlias, cannas) stored in a cool, dry location
• ✓ Protect any tender shrubs or perennials with additional mulch if temperatures drop severely

What NOT to Start in January Zone 6a

Just as important as knowing what to start is knowing what to leave alone.

Starting crops too early is one of the most common mistakes in Zone 6a — you end up with large, root-bound seedlings sitting under lights for weeks longer than ideal, and they rarely perform as well as properly timed transplants.

Do not start these in January in Zone 6a Pennsylvania:

• Tomatoes — Start 6–8 weeks before last frost. That puts your tomato start date in mid to late March. January tomato seedlings will be massive and stressed by April.
• Peppers — Can go a little earlier than tomatoes (8–10 weeks), so late February/early March is the window.
• Aubergine / Eggplant — Early March start is ideal.
• Cucumbers, courgettes, squash — These are fast growers. Start just 3–4 weeks before last frost in late March/early April.
• Beans — Direct sow outdoors only after all frost risk has passed. Never start indoors in January.
• Basil — Cold-sensitive and fast growing. Start in late March at the earliest.

A Note on Grow Lights for January Seed Starting

Pennsylvania in January gets approximately 9–10 hours of natural daylight, and most of it is weak and low in the sky. A windowsill alone will not give seedlings the intensity they need — you will end up with pale, leggy plants that struggle all season.

A basic T5 or LED grow light positioned 2–4 inches above seedlings for 14–16 hours a day makes an enormous difference. Set them on a timer so you do not have to think about it. This single investment is the biggest upgrade a home seed-starter can make.

Zone 6a Pennsylvania January Planting Summary

What Comes Next: February in Zone 6a Pennsylvania

February brings more seed-starting activity as the season builds momentum. Peppers, aubergine, and the first brassicas (broccoli, cabbage, cauliflower) come onto the agenda.

Cold frames can be sown with spinach and salad leaves by late February as soil temperatures begin to edge upward.

But that is next month. For now, get your onions and leeks in their trays, your seed order placed, and your grow lights on a timer. January done well is the foundation for everything that follows.