Starting nasturtiums from seed indoors is a project that rewards a little patience, and this guide walks you through it from the first seed to a plant that is ready for the garden. Nasturtium seeds are big, round and easy to handle, and they germinate in 7–14 days. They have a hard coat, so soaking them in warm water overnight before sowing speeds things up — or nick the coat gently with a file. Sow them about half an inch deep. The key fact about nasturtiums is their taproot: like sunflowers, they hate having it disturbed, so they are usually direct-sown where they will grow. Indoor starting is a limited option. If you do start them indoors, use peat pots or soil blocks, sow only 2–3 weeks before your last frost, and transplant while the seedling is still small.
Nasturtiums belong to the Tropaeolaceae family, need full sun, and grow best in soil with a pH of 6.0–7.5. Seeds take 7–14 days to germinate, plants take 50–60 days to reach maturity, they are tender — frost will damage or kill it, and they are usually spaced 8–12 inches apart.
Nasturtiums at a glance
| Growing factor | What nasturtiums need |
|---|---|
| Days to germination | 7–14 |
| Sowing method | direct sow |
| Days to maturity | 50–60 |
| Soil pH | 6.0–7.5 |
| Spacing | 8–12 inches apart |
| Sun | full sun |
| Cold tolerance | tender |
When to start nasturtiums seeds indoors
The short answer: start nasturtiums seeds indoors 2–3 weeks before your last frost at most — and only in peat pots or soil blocks.
For nasturtiums, the timing that matters is the direct-sowing date. Sow outdoors once the soil has warmed and frosts are over: late spring in Zones 3–5, mid-spring in Zones 6–8, early spring in Zones 9–10. In frost-free Zones 11–13, sow for much of the year; check your local extension service for the best local window. If you start indoors, sow in peat pots just 2–3 weeks before your last frost — any earlier and the taproot will be pot-bound. Use the frost date calculator to find your last frost date.
Frost dates vary by microclimate, so confirm your dates with your local extension service, and use the last first frost lookup to check your own frost dates before you sow.
What you need
- Fresh nasturtiums seed — check the packet date; test old seed with the seed viability checker.
- Seed starting mix — fine, sterile and free-draining. Do not use garden soil.
- Containers — cell trays, small pots or soil blocks, cleaned before use.
- A tray and a clear lid — to hold humidity until germination.
- Labels and a marker — you will not remember what you sowed in three weeks.
- A spray bottle — for gentle watering that does not wash small seeds away.
- A grow light — a sunny windowsill is rarely enough. Our grow light calculator helps you set it up.
- Potting mix for potting on — seedlings outgrow seed mix once they have true leaves.
Step by step: starting nasturtiums seeds indoors
Before the steps, the honest answer first: nasturtiums are usually direct-sown, not started indoors. The steps below cover the limited indoor option — in peat pots or soil blocks that go into the ground whole — and then the direct-sowing method that most gardeners use. If you only follow one set of steps, follow the direct-sowing ones.
- Fill containers with damp seed mix. Moisten the mix first — it should feel like a wrung-out sponge. Fill to just below the rim.
- Sow the seed. Soak the seeds in warm water overnight first, then sow one seed per peat pot or soil block, about half an inch deep, and cover with mix. Use peat pots or soil blocks only — nasturtiums hate root disturbance.
- Label everything. Write the crop and the sowing date on the label now, not later.
- Cover to hold humidity. Put a clear lid or cling film over the tray so the surface stays moist.
- Keep warm and wait. Nasturtiums seeds take 7–14 days to germinate — do not throw the tray away after a week. The germination time calculator gives you a realistic window.
- Remove the cover as soon as seedlings appear. A closed, humid tray is where damping off starts.
- Give light immediately. Move the tray under a grow light or to your brightest window, with the light 2–3 inches above the leaves.
- Water from below. Stand the tray in a shallow dish of water, let the mix soak it up, then drain.
- Thin to one seedling per cell. Snip the weaker seedling off at soil level — do not pull it out, or you will disturb the roots of the one you keep.
- Pot on when the roots fill the cell. Once seedlings have two sets of true leaves, move each one into a larger pot of potting mix. Handle seedlings by their leaves, never by the stem.
Direct sowing instead — the normal method for nasturtiums: wait until the soil has warmed and frosts are over, sow where the plants will grow, and keep the soil moist until seedlings are up. Thin to the final spacing once seedlings have two sets of true leaves.
Light for seedlings
Light is the single biggest difference between sturdy seedlings and leggy ones. A windowsill that looks bright is often far too dim for a seedling, because glass cuts light and late-winter days are short. Give nasturtiums seedlings 14–16 hours of light a day from a grow light hung 2–3 inches above the leaves, raising it as the plants grow. Our grow light calculator helps you set it up. Seedlings that stretch tall, pale and thin need more light — move the light closer at once.
Watering seedlings
Nasturtiums have moderate water needs. Keep the mix evenly moist, never soggy and never bone dry: check daily by touching the surface, and water if it feels dry. Water from below when you can, by standing the tray in a shallow dish for ten minutes and then draining it, so the surface stays dry and the roots grow down. Our watering calculator gives a starting point for amounts. Use room-temperature water and water in the morning. The two mistakes that cause most seedling losses are overwatering, which rots roots and invites damping off, and letting trays dry out completely, which kills fine roots in hours.
Hardening off
Hardening off is the week of gradual exposure that turns a soft indoor seedling into a plant that can survive outdoors. Start 7–10 days before planting out: put seedlings outside in a sheltered, shady spot for an hour or two on day one, then leave them out a little longer each day, with more sun and wind, until they spend full days outside. Do not leave them out overnight until the last couple of days, and bring them in if frost is forecast — nasturtiums are tender, so a frosty night can undo weeks of work.
Transplanting outdoors
Nasturtiums are direct-sown for a reason, so transplanting is the risky part. If you started in peat pots or soil blocks, plant the whole pot or block into the ground after the last frost, without pulling the seedling out or disturbing the roots. Space plants 8–12 inches apart in full sun, in soil with a pH of 6.0–7.5. Like cosmos, nasturtiums flower better in poor soil — rich soil gives you leaves and few flowers — so do not feed them. Our plant spacing calculator works out your layout, and the companion planting finder suggests neighbours — lavender, marigolds, garlic and chives all grow well with nasturtiums.
Transplant on a cloudy day or in the evening, water each plant in well, and mulch lightly around the base. Check daily for the first week — a seedling's roots are still shallow.
Common problems and fixes
| Problem | Likely cause | Fix |
|---|---|---|
| Seeds rot without sprouting | Cold, wet soil | Wait until the soil is warm; soak seed overnight and sow in warmth |
| Seedlings wilt after transplanting | Taproot disturbed | Start in peat pots or soil blocks and plant the whole thing |
| Lots of leaves, few flowers | Soil too rich | Do not feed nasturtiums — poor soil gives more flowers |
| Blackfly on young plants | Aphids love nasturtiums — they are often grown as a trap crop | Spray off with water, or leave them to draw aphids away from other crops |
Watch for pests once plants are outdoors: the common pests of nasturtiums are aphids, slugs, cutworms. Check plants weekly, looking under leaves as well as on top, and act early. Our pest identifier helper can help identify what you are seeing, and the plant disease checker helps narrow down leaf and stem problems.
Transplant shock
Nasturtiums grow a taproot that goes straight down, and once it is disturbed the plant sulks for weeks.
All leaves and no flowers
Nasturtiums flower best when they are slightly hungry, just like cosmos.
Aphids on nasturtiums
Aphids love nasturtiums, and many gardeners plant them deliberately as a trap crop to draw aphids away from vegetables.
Frequently asked questions
Should I start nasturtium seeds indoors?
Usually no. Nasturtiums are direct-sown because their taproot hates being disturbed. Start indoors only in peat pots or soil blocks, 2–3 weeks before your last frost.
Should I soak nasturtium seeds before sowing?
Yes, soaking in warm water overnight softens the hard seed coat and speeds germination. You can also nick the coat gently with a file.
How long do nasturtium seeds take to germinate?
Nasturtium seeds germinate in 7–14 days in warm, moist soil. Soaking beforehand helps them sprout faster and more evenly.
Why are my nasturtiums all leaves and no flowers?
The soil is too rich or you fed them. Nasturtiums flower best in poor soil — stop feeding and they will bloom.