January Seed Start Playbook

January can feel quiet outside, but inside a greenhouse it is the month that sets the tone for spring. With a simple plan you can start the right crops for your area, keep germination steady, and build a two week rhythm that brings strong seedlings right when you need them. This guide keeps it practical and neighborly so you can get moving today.

What you can start in January by climate band

Use these groups as a friendly compass. Local frost dates rule, so adjust a little earlier or later for your site. If you are unsure, begin modestly and scale up in February.

Cold winter band
high mountain and northern areas with long freezes

• Onions from seed

• Leeks

• Shallots

• Early brassicas for later transplant such as cabbage and broccoli

• Slow herbs such as parsley

• Microgreens for quick harvests
  Hold peppers and tomatoes until late February to early March unless you have strong light and warm nights.

Moderate winter band
many inland valleys and coastal areas with regular frost but milder days

• All cold band crops above

• Hardy greens for transplant such as lettuce, spinach, kale, pak choi

• Early sweet peas in cell trays

• Celery and celeriac
  Tomatoes and peppers can begin late in the month if you can give ten to twelve hours of strong light and a warm bench.

Mild winter band
warm coastal pockets and low desert zones

• All crops above

• Trial a first sowing of tomatoes and peppers in the last week of January with real light and steady bottom warmth

• Basil in a small test tray if your nights inside the greenhouse are holding above fifty

Germination temperature targets

Seed wants the right soil temperature more than anything. A small soil thermometer and a heat mat with a thermostat make January work simple.

• Onions leeks and shallots
  Best around sixty to seventy

• Parsley and slow herbs
  Sixty five to seventy five

• Lettuce and spinach
  Fifty five to sixty five

• Brassicas such as cabbage and broccoli
  Sixty to seventy five

• Peas
  Fifty to sixty

• Celery and celeriac
  Seventy to seventy five

• Tomatoes and peppers if you start tests late month
  Seventy five to eighty five for quick germination

Set the mat thermostat to the lower end of each range and be patient. Warmer is not always better for plant quality.

Light that seedlings can use

January sun is precious. Clean the inside of your glazing so every ray reaches the bench. If you add lights, keep them close enough to prevent stretch but high enough for even spread.

• Aim for ten to twelve hours of total light

• Keep most LED fixtures eight to twelve inches above small starts

• Raise lights as plants grow to maintain an even wash

• Spin trays a quarter turn each visit for uniform growth

If seedlings reach and pale, you need either more light or a cooler air temperature so growth slows to match light.

The two week rhythm that works

A steady cadence beats a burst of effort. This simple loop keeps trays moving without filling every surface.

Week one
Sow slow and slow to medium crops

• Onions leeks parsley celery

• One tray of lettuce or spinach for an early transplant block

• One small test tray of a new variety you are curious about

Week two
Tend and thin then sow again

• Thin trays to one strong seedling per cell

• Bottom water and check roots

• Sow a second tray of lettuce or spinach and a tray of cabbage or broccoli

Repeat the pattern. Every two weeks you sow a small set and tidy the previous set. By late February you will have a clean staircase of transplants at the perfect ages rather than a single tide of overgrown starts.

Soil mix and containers

Seedlings thrive in a light consistent medium that drains well yet holds moisture.

• Use a quality seed starting mix with fine texture

• Moisten the mix before filling cells until it holds shape when squeezed and crumbles when tapped

• Choose cell trays for crops that dislike root disturbance such as lettuce and brassicas

• Use open flats for onions and leeks then slice into blocks at potting up

• Always label with crop variety and sow date right away

Watering in winter light

Overwatering is the most common January mistake. Roots want moisture with air, not a swamp.

• Bottom water so cells drink from below and foliage stays dry

• Pour off excess after ten to fifteen minutes so trays never sit in water

• Let the top of the mix dry slightly between waterings

• Water in the morning so leaves dry before evening

If algae forms on the surface, you are likely watering too often or starving the tray of airflow.

Warmth for roots and comfort for you

You do not need to heat the whole greenhouse. Target the bench.

• Use a heat mat with a reliable thermostat under the trays that need it most

• Add simple insulation under the mat such as a foam board offcut to limit heat loss

• Close vents well before sundown to hold late day warmth

• Add thermal mass such as dark water jugs along the north wall for smoother night temperatures

Pair gentle warmth with fresh air at midday on bright days so humidity stays in check.

Potting up without stalling growth

The first pot up is when many seedlings stall. Make it smooth.

• Pot on when roots just knit the plug and tips are visible at the drain holes

• Move into a slightly larger cell or small pot rather than a big jump

• Water in with room temperature water

• Keep light strong for a few days after potting to prevent stretch

Onions and leeks can be trimmed to three inches once to keep them tidy and stocky before transplanting.

Hardening plan for late winter transplants

Even with a greenhouse, transplants need a short acclimation to cooler air and wind.

• One week before transplanting, reduce heat mat use for that tray

• Give trays a daily taste of cooler air at a cracked door or vent for an hour or two

• Avoid sudden full sun and wind exposure on day one

• Plant out on a day that is calm and overcast if possible

A small cold frame or low tunnel inside the greenhouse can act as a training ground in late February and March.

Troubleshooting at a glance

• Seedlings are tall and pale
  Light is weak or too far away or night air is too warm

• Leaf edges are burned or yellow
  Fertility is too strong or salts are building from frequent small waterings

• Mold on soil surface
  Too much moisture and not enough air movement

• Slow or spotty germination
  Soil temperature is outside the ideal range or seed is old

Keep a small fan on low to move air gently across trays. That one habit prevents many problems.

A simple weekly checklist

Post this at the bench so you can stay on track without thinking hard.

• Clean light lenses and wipe a small window pane

• Sow one or two trays

• Thin last week’s trays

• Bottom water and drain

• Note max and min temperatures

• Spin trays and raise lights if needed

• Record what you did and what you noticed

Bringing it together

January rewards growers who start small and steady. Choose crops that truly belong to your climate band. Give seeds their preferred soil temperature. Keep light close and even. Water from below and let air move. Follow a two week sow and tend rhythm so you always have the right age seedlings ready to go.

With that playbook you will meet February and March with confidence. Trays will be stocky. Roots will be white and full. Transplants will hit the ground running. Most important, your greenhouse will feel calm and organized rather than rushed. If you would like, I can turn this plan into a printable calendar and a one page bench card with temperature and timing targets for your most common crops.