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How to sow petunia seeds

Updated: 21 October 2026

You have a packet, a windowsill or a propagator, and a date you hope to plant out. Petunia seed is dust-fine, it needs light, and a late frost will kill the plants. How to sow petunia seeds in the UK is a surface sow under cover in early spring, not a May scattering into a cold basket. This page is the method. The [Petunia seeds comparison](/hub/herb-seeds/petunia-seeds) is the product grid if you still need a packet. Peter F. used Thompson & Morgan, BBC Gardeners' World, the RHS, and the Met Office. He did not run a germination trial. A qualifying purchase can earn a commission at no extra cost to you.

You have a packet, a windowsill or a propagator, and a date you hope to plant out. Petunia seed is dust-fine, it needs light, and a late frost will kill the plants.

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Just Seed Just Seed Double Pirouette Blue

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Just Seed Just Seed Double Pirouette Blue
7.8/10about £2.25
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Just Seed Just Seed Double Pirouette Red and White

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Just Seed Just Seed Double Pirouette Red and White
7.4/10about £2.25
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Damavandpro Damavandpro nana compacta mixed

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Damavandpro Damavandpro nana compacta mixed
4.0/10about £3.50
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On this page
  1. 1.When to sow in the UK
  2. 2.How to sow so the seed can germinate
  3. 3.After they are up
  4. 4.When they can go outside

When to sow in the UK

Start under cover in early spring, often from February into March if you can hold a steady warmth. The plants are grown as annuals in Britain. A packet that says hardy perennial does not change that. RHS H2 on trailing cultivars checked for this subject means about 1–5°C, not a freeze. Count backwards from the plant-out date where you live. You need time to germinate, prick out, grow on, and harden off for about two to three weeks. If you only decide in the second half of May, buy plants for this year. If you have not chosen a packet yet, the petunia seeds buying guide is the check before these steps.

How to sow so the seed can germinate

Use a small tray of moist peat-free seed compost. Thompson & Morgan and BBC Gardeners' World both say sow on the surface and do not cover. Press the seed on gently, or sieve a dust of vermiculite, not a blanket of soil. Damavand nana compacta tells you to sow 1 inch deep. Do not follow the inch. Aim for about 18–24°C at seed level, including at night. Vent a cover once you see green. Sow thinly. A pellet is one seed in a coat: space it and let the compost soften it. Do not soak the coat off in a cup. Just Seed allows about two to three weeks for Pirouette. No germination percentage is verified for these packets.
Just Seed Just Seed Double Pirouette Blue

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Just Seed Just Seed Double Pirouette Blue
7.8/10about £2.25
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After they are up

Prick out when each seedling has two true leaves. Hold a leaf, not the stem, and set each plant into its own small pot of peat-free compost. After pricking out, Thompson & Morgan's note is the switch people miss: below about 10°C encourages side shoots and delays flowering, and above about 15°C pushes one main stem and a leggy plant. A cool, bright porch beats a hot dark shelf. Leggy seedlings are usually short of light. If nothing appears after about three weeks, check moisture and night temperature. A dried surface for a single afternoon is enough to stop dust seed. An empty packet is a returns question on amazon.co.uk.

When they can go outside

Harden off for about two to three weeks. Put the pots outside by day, bring them in at night, and do not move them from a propagator to a frosty patio in one afternoon. The RHS uses nights reliably above about 7°C as the tender-plant signal, and tells you to check the forecast. As a rule of thumb, plant out from the second half of May in southern England and south Wales, later as you go north, and toward mid June in the Highlands and northern Scotland. The Met Office May note says the same for tender bedding, including petunias, and only if frost has passed. If the forecast shows frost, they stay indoors.

Common questions

Do I cover petunia seed with compost?
No. Sow on the surface of moist compost so the seed gets light. A press with a board or a finger is enough contact. Thompson & Morgan and BBC Gardeners' World both say do not cover. Ignore a marketplace care line that says 1 inch deep. A thick cover is a common reason a full packet appears to do nothing.
What temperature do I need, including at night?
About 18–24°C at seed level, held overnight as well as in the afternoon. A room that drops when the heating clicks off is too uneven. After pricking out, cooler growing-on, below about 10°C, encourages branching, while a steady heat above about 15°C hurries a single stem. Measure near the tray, not at the ceiling.
How long until they germinate?
About one to three weeks in the sources used here. Thompson & Morgan's retail note is often one to two weeks. Just Seed allows two to three weeks for Pirouette. That is a window, not a promise, and it assumes light, moisture, and the temperature above. No percentage is verified for these amazon.co.uk packets. Flowers come much later than the first green leaf.
When do I prick them out, and should I then keep them warm?
Prick out at two true leaves into individual pots. You do not keep them in the same high germination heat. Cooler, bright conditions after that encourage side shoots. Above about 15°C the main stem races and the plant looks thin. Handle a leaf, not the stem, and do not leave them in a sealed lid once they are up.
When can they go outside where I live?
After frost, following about two to three weeks of hardening off, on nights reliably above about 7°C. As a rule of thumb the RHS points to the second half of May in southern England and south Wales, later further north, and mid June in the Highlands and northern Scotland. Check a forecast. A mild fortnight does not cancel a frost pocket.
Why are seedlings long and floppy?
They are usually short of light, or they stayed too warm after germination. Move the tray to a bright window and drop the temperature toward the cooler growing-on range once the true leaves are there. A sealed lid and a thick sowing make floppy, collapsing seedlings as well. Sowing an inch deep, as one listing suggests, does not fix a thin stem. It stops the seed seeing light.
Will they survive winter outside if the packet says perennial?
Do not plan on it. In the UK these plants are treated as frost-tender annuals. H2 means roughly 1–5°C, not a hard frost, and a hardy perennial title is not a winter plan. A USDA zone on a listing is the wrong map. Compost spent plants at the end of the season unless you are keeping a pot frost-free under cover as an experiment, not as a promise.

Related pages

Petunia seeds for hanging baskets

Trailing or upright petunia seeds

Petunia seeds

Best petunia seeds in this amazon.co.uk comparison

Petunia seeds UK: what your packet should show you

Sources

Public UK sources named below, plus the Just Seed variety page and the Surfinia brand page. amazon.co.uk prices, stars and review counts are from the twelve listings in this comparison. Page updated 21 October 2026.

  1. Thompson & Morgan, Petunias (retrieved 21 October 2026)
  2. BBC Gardeners' World, How to grow petunias (retrieved 21 October 2026)
  3. RHS, When to plant out tender plants (retrieved 21 October 2026)
  4. RHS, Hanging baskets (retrieved 21 October 2026)
  5. Seed Marketing Regulations 2011 (retrieved 21 October 2026)
  6. GOV.UK, Issuing plant passports (retrieved 21 October 2026)
  7. Met Office, Gardening tips for May (retrieved 21 October 2026)
  8. Just Seed, Pirouette red and white (retrieved 21 October 2026)
  9. Surfinia brand (retrieved 21 October 2026)