Leeks are one of the longest-season vegetables you can grow, and one of the most forgiving once they are in the ground. Starting them indoors is standard practice — the seed is small, the season is long, and a head start is what gets you thick stems by autumn.
Leeks belong to the Amaryllidaceae family, need full sun, and take 90–120 days to mature, with seed germinating in 7–14 days. That combination — a long season and fairly quick germination — shapes every decision in this guide, from when you sow to whether indoor starting is even worth it. Leeks take 90–120 days to mature, so an indoor start of 10–12 weeks is what makes the difference between pencil-thin stems and proper leeks. The seedlings look like blades of grass and are easy to underestimate, but they are tougher than they look — you can even trim their tops to keep them stocky while they grow on indoors.
Leeks at a glance
| Growing factor | What leeks need |
|---|---|
| Days to germination | 7–14 |
| Sowing method | both |
| Days to maturity | 90–120 |
| Soil pH | 6.0–7.0 |
| Spacing | 6–8 inches |
| Sun | full sun |
| Cold tolerance | hardy |
| Water needs | moderate |
| Family | Amaryllidaceae |
When to start leeks seeds indoors
In Zones 3–6, sow leeks indoors 10–12 weeks before your last frost for a summer and autumn harvest. In Zones 7–9, sow in late winter for summer leeks, or in late summer for leeks that overwinter. In Zones 10–13, sow through the cool season. Leeks are hardy and can stand in the ground through winter in many zones.
Timing in brief: 10–12 weeks before your last frost. Work backwards from your last frost date using our seed-starting calculator and the frost-date calculator, and confirm the timing with your local extension service. Zones 11–13 are frost-free, so never plan around a frost date there — the calendar in those zones is about heat and rain instead. If you are unsure of your zone, the hardiness-zone finder will confirm it.
What you need
- Fresh leeks seed — check the packet date, and test old seed before you rely on it
- Seed trays or cells, plus peat pots or soil blocks if you are starting a crop that dislikes root disturbance
- Fine, sterile seed-starting mix — not garden soil, which is too heavy and carries disease
- A spray bottle or a tray for bottom watering
- Labels and a marker — leeks seedlings all look alike at first
- A grow light, or the brightest windowsill you have
- A heat mat, if you are starting a warmth-loving crop or your room runs cool
- Clean water, and a small fan for air circulation once seedlings emerge
Keep the seed packet as your reference for variety, sowing depth, and days to maturity. Wash reused trays in hot soapy water first — old compost is where damping-off fungi live.
Step by step: starting leeks seeds indoors
Sow leek seed 1/4 inch deep in trays or cells — you can sow several seeds per cell and separate them later, or sow thickly in a tray and prick out. Germination takes 7–14 days. Trim the tops back to about 4 inches if seedlings get floppy; it keeps them stocky.
- Fill your containers. Fill trays, cells, or peat pots with moist seed-starting mix, damp like a wrung-out sponge. Water dry mix before you sow, not after.
- Sow at the right depth. Sow at the depth on the packet and no deeper — seed sown too deep runs out of energy before it reaches the surface.
- Label everything. Write the variety and sowing date on every tray.
- Water gently. Mist the surface or water from below so seed is not washed away. Cover the tray to hold moisture in, and remove the cover the moment seedlings appear.
- Keep it warm and bright. Seed germinates in 7–14 days in suitable conditions — check daily, and move the tray into light the moment green appears.
- Thin to the strongest. If two seedlings share a cell, snip the weaker one at soil level — pulling disturbs the roots of the one you keep.
- Pot on when ready. Move seedlings into individual pots once they have two or three true leaves, handling them by a leaf, never the stem.
- Feed lightly. Seed mix has almost no nutrients, so once seedlings have true leaves, feed at half strength with a balanced liquid feed every week or so.
For direct-sown crops, the outdoor version is simpler: sow where the plant will grow, keep the bed moist until germination, and thin to 6–8 inches apart. The soil pH for leeks should be 6.0–7.0.
Light for seedlings
Leek seedlings need strong light to stay stocky. Give them a bright windowsill at minimum, and a grow light a few inches above the leaves is better — 14–16 hours a day.
Give seedlings 14–16 hours of light a day. A south-facing windowsill can work in spring, but most seedlings do better under a grow light hung a few inches above the leaves and raised as they grow — if the light is too far away, seedlings stretch and go leggy, and leggy seedlings rarely recover. Our grow-light calculator can help with hanging height and hours. Give them a dark night too; do not leave lights on around the clock.
Watering seedlings
Keep compost evenly moist during germination (7–14 days) and while seedlings grow on. Leeks need moderate water; they dislike drying out, but they also dislike sitting wet.
Water seedlings from below whenever you can: stand the tray in a shallow dish for ten minutes, let the mix soak it up, then drain it away. Top watering washes seed around and keeps the surface wet, which invites damping off. Keep the mix evenly moist — never soggy, never dry — and check daily, because a small cell dries fast under lights and a dry spell during the 7–14-day germination window can stall the seed. Once seedlings have true leaves, let the surface dry slightly between waterings. Our watering calculator gives a starting point. Use room-temperature water — cold water shocks young roots.
Hardening off
Harden leeks off over 7–10 days. They are hardy, so they can go out while nights are still cool, but a gradual move avoids checking their growth.
Hardening off means getting indoor seedlings used to the outdoors gradually, over 7–10 days. Start with a few hours in a sheltered, shady spot, then lengthen the time outside each day and introduce more sun and wind. Bring trays in at night for the first few days, especially if frost threatens. Seedlings put straight outside without hardening off wilt, scorch, or stop growing for weeks — the shock undoes all the careful indoor work.
Transplanting outdoors
Set leeks 6–8 inches apart, dropping seedlings into dibber holes so the white stem blanches as it grows — the hole fills in naturally as you water. Water in well and mulch. For longer white stems, earth up the plants as they grow.
Leeks grow best in full sun and a soil pH of 6.0–7.0, so choose the sunniest suitable spot and test the soil before you plant. Good companions nearby include marigolds, basil, onions. Space plants 6–8 inches apart — crowding is one of the commonest causes of small harvests. Our plant-spacing calculator works out how many plants fit your bed. Plant on a cloudy day or in the evening, water in thoroughly, and mulch to hold moisture.
Common problems and fixes
| Problem | Fix |
|---|---|
| Aphids | Check plants regularly, remove by hand, and cover young plants with insect mesh. |
| Slugs | Spray off with water and encourage beneficial insects. |
| Damping off | Use fresh sterile seed mix, water from below, and give seedlings good air circulation. |
| Leggy seedlings | Move grow lights closer — a few inches above the leaves — and give 14–16 hours of light a day. |
| Seed not germinating | Check the seed is fresh and the compost is moist; leeks seed takes 7–14 days to germinate. |
Thin, floppy seedlings
Too little light. Move lights closer and trim tops to keep seedlings stocky.
Damping off
Seedlings collapse at soil level. Use fresh sterile mix, water from below, and improve air flow.
Slugs on young plants
Slugs love young leeks. Use barriers and keep the bed clear of hiding places.
Frequently asked questions
Can I sow leeks directly outdoors?
Yes — leeks can be direct-sown, but the season is so long (90–120 days) that an indoor start usually gives much thicker stems. Direct-sown leeks suit mild zones with a long season.
Should I trim leek seedlings?
Yes, trimming the tops back keeps them stocky and stops them flopping. It does not harm the plant — leeks regrow quickly.
How do I get long white stems?
Plant deep in dibber holes and earth up as the plant grows. The buried part stays white and tender.
How long do leeks seeds take to germinate?
Leeks seed takes 7–14 days to germinate in suitable conditions. Keep the compost moist and at room temperature, and use fresh seed — old seed germinates slowly or not at all. Our germination-time calculator can help you work out the timing for your sowing.
Can I use old leeks seed?
Check it first. Seed viability falls with age and poor storage, so test a few seeds on damp kitchen paper before sowing a whole tray. Our seed-viability checker walks you through the test.