Sow nasturtium seeds straight where they will grow, once your last spring frost has passed and the soil feels warm. Push each seed 1/2 to 1 inch deep, keep the soil lightly moist, and look for sprouts in 7 to 14 days. Start indoors only 2 to 4 weeks early, in pots you can plant whole. Then keep a few notes on each sowing so you know which method to repeat next year.
Nasturtiums are a bit contrary at sowing time. The big seeds are easy to handle and quick to sprout. But the seedlings sulk when their roots are disturbed, and plants in rich soil turn into a mound of leaves with hardly a flower.
Most nasturtium problems are decided in the first 3 weeks: where the seed went, how warm the soil was, and how wet it stayed. Each of the 13 steps below deals with one of those, which makes it easier to tell a timing problem from a soil problem later.
Before sowing, check your last frost date and pick a spot with at least 6 hours of sun and ordinary, unfertilized soil. Poor soil is fine here. Honestly, it’s what you want.
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13 nasturtium seed-sowing steps from packet to planting
1. Set a local sowing window for nasturtium
Direct sow once your last frost has passed and the soil is around 60 to 65°F, usually 1 to 2 weeks after the frost date. They’re tender. One frosty night is enough to blacken the leaves.
In zones 9 to 11, you can sow in fall or late winter for cool-season flowers, since nasturtiums often slow down or stop blooming in midsummer heat above the mid 80s.
2. Use the packet depth for nasturtium
Nasturtium seeds are wrinkled and about the size of a chickpea. Push each one 1/2 to 1 inch into the soil with a fingertip and firm the soil over it; darkness helps them germinate.
Space them 8 to 12 inches apart for bushy types and about 12 inches for trailing ones, or sow 2 seeds per spot and thin later. An optional soak of 8 to 12 hours in lukewarm water can speed sprouting by a few days.
3. Hold the germination temperature steady for nasturtium
Soil between about 55 and 65°F is plenty, and most seeds come up in 7 to 14 days. No heat mat needed; that’s for peppers and basil.
Cold soil below 50°F is the bigger risk: the seed swells, sits, and rots. If a spring cold snap arrives right after sowing, expect a slower start and do not resow until 3 weeks have passed.
4. Keep the seedbed evenly moist for nasturtium
Water the row well at sowing, then keep the top inch damp but not muddy until the round seed leaves appear. A fine-rose can soaks in gently without crusting the surface over the seed.
Once seedlings are up, let the top inch dry between waterings. Nasturtiums kept constantly wet tend to grow lush leaves and fewer flowers.
5. Give emerging nasturtium seedlings strong light
Outdoors, sow in full sun, at least 6 hours, with afternoon shade only where summers run very hot. Plants in deep shade grow long, weak stems with sparse flowers.
Indoor starts need 14 to 16 hours under lights hung 3 to 4 inches above the leaves. Nasturtium seedlings stretch fast on a windowsill and flop once moved outdoors.
6. Thin nasturtium seedlings before crowding
When both seeds in a spot come up, snip the weaker one at the soil line once the first true leaves open, about 2 weeks after sowing. Pulling it tears the roots of the keeper.
Crowded plants shade each other and stay damp longer. Aphids love that soft new growth.
7. Label the variety and sowing date for nasturtium
Mounding nasturtiums stay about 12 inches tall, while trailing and climbing types can run 6 feet or more. The seeds look the same, so label each spot with variety and sowing date.
The date matters too: flowers usually show up 6 to 8 weeks after sowing.
8. Plan for nasturtium root disturbance
Nasturtiums resent having their roots pulled apart, which is the main reason direct sowing usually beats transplants. If you start them inside, sow one seed per 3 to 4 inch biodegradable or paper pot.
Plant the whole pot so the roots never leave the soil they grew in. Seedlings shaken out of shared trays often wilt and stall for 1 to 2 weeks.
9. Harden nasturtium seedlings off gradually
Indoor nasturtiums need 5 to 7 days of hardening before they go out. Start with 2 hours in bright shade and add a few hours each day.
Those soft, round leaves scorch fast in sudden sun and tear in wind. If you see pale, crisp patches, back off for a day or two.
10. Transplant nasturtium at the right stage
Move indoor nasturtiums outside when they have 2 to 3 true leaves and nights stay above 50°F, usually 3 to 4 weeks after sowing. Older seedlings in small pots get root-bound and take longer to recover.
For containers, one bushy nasturtium fills an 8 to 10 inch pot, and a 24 to 30 inch window box takes 3 or 4. Use plain potting mix without added fertilizer.
11. Watch young nasturtium for damping-off
Damping-off is rarer with nasturtiums than with tiny-seeded flowers. It still happens in cold, soggy pots. Look for a seedling that collapses with a dark, thin stem at the soil line.
Use fresh mix in clean pots, water from below, and let the surface dry slightly between waterings. Outdoors, slugs are the more likely culprit when seedlings vanish overnight.
12. Keep one backup sowing for nasturtium
Save a few seeds and sow a second round 3 to 4 weeks after the first. That fills any gaps from slugs or cold soil, and you’ll have fresh plants when the first batch tires out in summer heat.
In short-season gardens, sow the backup no later than about 10 weeks before first fall frost so it still blooms.
13. Record days to emergence for nasturtium
Count the days from sowing to the first round seed leaf. In soil near 60 to 65°F, most nasturtiums break the surface in 7 to 14 days.
Over 3 weeks usually means cold soil or old seed. Depth is rarely the culprit. Write that number down first.
Set the nasturtium sowing window from local conditions
Nasturtiums appear on the RHS School Gardening sheet of flower seeds that are easy to sow as a simple choice for direct sowing outdoors. In US gardens, treat them as frost-tender and let your frost dates set the calendar.
| Your situation | Nasturtium timing |
|---|---|
| Direct sowing | 1 to 2 weeks after last frost, soil about 60°F |
| Indoor start | 2 to 4 weeks before last frost, in plantable pots |
| Planting out | After last frost, nights above 50°F |
| Last sowing | About 10 weeks before first fall frost |
| Zones 9 to 11 | Fall to late winter for cool-season bloom |
In frost-free areas such as coastal California, nasturtiums self-sow freely and can spread into nearby wild spaces. Pull volunteers you do not want while they are small. If you are planning cool-season greens in the same bed, the guide to sowing lettuce seeds sets that earlier timing.
Control depth, moisture, temperature and light
With nasturtiums, all four come down to the same idea: keep it simple and a little lean.
- Depth: 1/2 to 1 inch, firmed so the seed sits in the dark.
- Moisture: damp until sprouts show, then drier between waterings.
- Temperature: soil around 55 to 65°F; avoid sowing into cold, wet ground.
- Light: 6 or more hours of sun outdoors, or 14 to 16 hours under lights.
Leave out compost-rich mixes and fertilizer. Nasturtiums in fertile soil grow big leaves and few flowers. A bag of sterile mix for indoor pots usually runs $8 to $15, and plain garden soil works fine for direct sowing.
Hot, dry summers are the real test. Nasturtiums often stop flowering above the mid 80s and recover when nights cool in fall. Where that happens, I’d give them afternoon shade and sow a fresh round in late summer instead of fighting the heat.
Pet note: the ASPCA lists nasturtium as non-toxic to dogs and cats, and the flowers and leaves are edible for people.
Move nasturtium seedlings without rushing them
There’s no prize for planting before the last frost. A seedling hit by a 32°F night turns translucent and black, and seed sown 2 weeks later in warm soil usually catches up.
Plant indoor starts in the evening or on a cloudy day, pot and all, at the same depth. Water well and again 3 days later, then only when the top inch is dry.
Nasturtiums sit well at the edge of a vegetable bed, trailing under taller crops. If cucumbers share that bed, the steps for sowing cucumber seeds follow the same after-frost timing.
Record emergence and improve the next batch
Think of it as a germination log for your nasturtium seeds. It follows each sowing from soil to sprouts to first flower, and it ends with one decision for next time: direct sow or start inside, earlier or later, leaner soil or the same.
| Trail entry | What to write |
|---|---|
| Variety | Mounding, trailing or climbing |
| Method | Direct sown or plantable pot |
| Sowing date | Days before or after last frost |
| Soil feel or reading | Cold, cool or warm |
| Soaked | Yes or no, hours |
| Days to emergence | First seedling and most of the row |
| Losses | Rot, slugs, wilt after transplant |
| First flower | Days from sowing |
| Leaf to flower balance | Mostly leaves or plenty of blooms |
| Decision | One change for next sowing |
Look back over it at midsummer. If direct-sown plants flowered within a week of the indoor ones, skip the indoor step next year. Lots of leaves and few flowers? That’s the soil being too rich, not a sowing mistake. Marigolds share the same after-frost window, and the marigold seed-sowing guide pairs well for a mixed edge.
If seedlings yellow and fail in the same bed two seasons in a row, a county extension office can check drainage and soil.
FAQ
When should I sow nasturtium seeds?
Sow nasturtiums outdoors after your last spring frost, once the soil is around 60°F. That is usually 1 to 2 weeks after the frost date. In zones 9 to 11, fall and late winter sowings work well too.
Should I start nasturtium seeds indoors?
Usually not. Nasturtiums sprout quickly outdoors and dislike root disturbance. If your season is short, start them only 2 to 4 weeks before last frost in plantable pots you set straight into the ground.
How deep should I sow nasturtium seeds?
Sow nasturtium seeds 1/2 to 1 inch deep and firm the soil. Keep it damp and look for sprouts in 7 to 14 days. If nothing appears after 3 weeks, check for cold soil or slugs before sowing again.
What temperature helps nasturtium seeds germinate?
Soil around 55 to 65°F suits nasturtiums, with sprouts in about 1 to 2 weeks. They do not need extra heat. Avoid sowing into soil below 50°F, where seed tends to rot.
How much light do nasturtium seedlings need?
Outdoors, give nasturtiums at least 6 hours of direct sun. Indoors, use 14 to 16 hours of light with the fixture close to the leaves. Long, floppy stems mean the light is too weak.
When can I move nasturtium seedlings outdoors?
Move them out after your last frost, when nights stay above 50°F and seedlings have 2 to 3 true leaves. Harden them off for 5 to 7 days first. Plant the whole pot to protect the roots.
Final thoughts
Nasturtiums ask for very little at sowing. Warm soil, a seed pushed in up to your first knuckle, ground that isn’t too rich. When they disappoint, it’s nearly always because they went in too early or got fed too much.
Your next step: once your frost date has passed, push 6 seeds into a sunny edge and write down the date and how the soil felt.
Last update on 2026-10-02 / Affiliate links / Images from Amazon Product Advertising API