Asparagus is the longest game in the vegetable garden: you sow seed this year and harvest properly two to three years later. Starting seed indoors is the cheapest way to build a bed, and the plants you raise will feed you for decades.
Asparagus belong to the Asparagaceae family, need full sun, and take 730–1095 days to mature, with seed germinating in 14–21 days. That combination — a long season and slow germination — shapes every decision in this guide, from when you sow to whether indoor starting is even worth it. Most gardeners buy asparagus crowns, which are one-year-old roots, but seed is far cheaper and gives you more plants. The trade-off is time: seed-grown asparagus needs a full season to build a root system, and you should not harvest spears until the second or third year. Think of the first year as root-building, not food-growing.
Asparagus at a glance
| Growing factor | What asparagus need |
|---|---|
| Days to germination | 14–21 |
| Sowing method | both |
| Days to maturity | 730–1095 |
| Soil pH | 6.5–7.5 |
| Spacing | 12–18 inches |
| Sun | full sun |
| Cold tolerance | hardy |
| Water needs | moderate |
| Family | Asparagaceae |
When to start asparagus seeds indoors
Start asparagus seed indoors 10–12 weeks before your last frost in Zones 3–8, so seedlings are sturdy by planting-out time. In Zones 9–10, sow in late winter. In frost-free Zones 11–13, start in the cool season. Wherever you are, the bed you prepare this year is for harvests years from now, so there is no rush — but there is also no shortcut.
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 asparagus 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 — asparagus 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 asparagus seeds indoors
Soak asparagus seed overnight, then sow 1/2 inch deep in deep pots — the seedlings make long roots quickly. Germination takes 14–21 days. Grow seedlings on in their pots for the whole first season if you can, feeding lightly, and plant them out the following spring.
- 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 14–21 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 12–18 inches apart. The soil pH for asparagus should be 6.5–7.5.
Light for seedlings
Asparagus seedlings are ferny and delicate, and they stretch fast in poor light. Give them a bright spot or a grow light a few inches above the leaves for 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 seed compost evenly moist while seed germinates over 14–21 days. Once growing, asparagus needs moderate water — it dislikes sitting wet, but young plants should never dry out completely.
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 14–21-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 seedlings off over a week before planting out, moving them outside for longer each day. Asparagus is hardy, so a light frost will not hurt established plants, but protect young seedlings from a hard freeze.
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
Plant asparagus 12–18 inches apart in a permanent bed — this bed will be there for twenty years, so dig it deep and enrich it with compost before you plant. Set seedlings at the same depth they stood in the pot, water in, and mulch. Do not harvest in the first year; let the ferns feed the roots.
Asparagus grow best in full sun and a soil pH of 6.5–7.5, so choose the sunniest suitable spot and test the soil before you plant. Good companions nearby include marigolds, basil, onions. Space plants 12–18 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; asparagus seed takes 14–21 days to germinate. |
Seedlings are thin and weak
Too little light or too small a pot. Give strong light and pot on into deep pots so the roots have room.
Spears are thin in year two
The plant was harvested too early or is underfed. Do not harvest in year one, and feed the bed with compost each spring.
Slugs on young shoots
Slugs love tender asparagus shoots. Use barriers, go out after rain, and keep the bed clear of debris.
Frequently asked questions
Is it better to grow asparagus from seed or crowns?
Crowns give you a harvest a year sooner, but seed is much cheaper and gives more plants. If you have patience, seed is the better value; if you want spears sooner, buy crowns.
How long until I can harvest asparagus?
Asparagus takes 730–1095 days to mature — plan on a light harvest in year two and a full harvest from year three. The wait is the price of a bed that lasts decades.
Do I cut the ferns back?
Leave the ferns alone all summer — they feed the roots. Cut them down only after they have yellowed in autumn.
How long do asparagus seeds take to germinate?
Asparagus seed takes 14–21 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 asparagus 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.