Dahlia Seed Grow Guide

Please note: Dahlia seeds do not grow true to the parent plant — every seedling will be genetically unique and produce its own distinct flower. If you want to replicate a specific variety, tubers are the reliable method. Growing from seed is an adventure: you never quite know what you’ll get.

Start indoors

6–8 weeks before last frost

Germination temp

20–24°C

Germination time

7–14 days

Days to bloom

100–120 days

Sun requirement

Full sun (6–8 hrs)

Plant out when

After all frost risk has passed

Step 1 — Germination: Paper Towel Method

This is our recommended starting point. It lets you identify viable seeds before committing them to trays.

1

Dampen two sheets of paper towelThey should be moist but not dripping — squeeze out any excess water.

2

Space seeds evenly on one sheetMake sure seeds are not touching each other. This prevents any damping off from spreading between seeds.

3

Fold the second sheet over the topPlace inside a zip-lock bag or sealed container to retain moisture, and store somewhere warm — a heat mat, top of the fridge, or warm windowsill all work well.

4

Check dailySeeds should show a small root within 7–14 days. Transfer to seed trays as soon as germination occurs — don’t let roots grow too long or they’ll tangle in the towel.
Tip: Any seeds that haven’t germinated after 2 weeks are unlikely to be viable. Focus your energy on the seeds that have sprouted.

Step 2 — Transferring to Seed Trays

Once a seed shows a small root, it’s ready to be moved into growing medium.

1

Fill cells with quality seed-raising mixUse a fine, well-draining seed-raising mix. Avoid mixes heavily amended with fertiliser at this stage.

2

Place germinated seed just below the surfaceGently press the seed about 5mm into the mix, root pointing down. Cover lightly and firm the surface.

3

Keep warm and brightMaintain temperatures of 20–24°C. Once shoots emerge, move trays close to a bright light source to prevent seedlings becoming leggy.

4

Water from the bottomSit trays in a shallow dish of water and allow the mix to wick up moisture. This prevents damping off and avoids disturbing delicate seedlings.
Tip: A thin layer of vermiculite over each cell helps retain surface moisture while allowing airflow, reducing the risk of damping off.

Step 3 — Growing On Indoors

Dahlias need 6–8 weeks of indoor growing before they’re ready for the garden. Keep conditions consistent and don’t rush.

1

Pot up when roots are establishedOnce seedlings have their first set of true leaves, move them into individual pots (at least 75mm). Do not allow plants to become root-bound.

2

Handle roots gentlyDahlias dislike root disturbance. Handle seedlings by their leaves rather than stems or roots when transplanting.

3

Provide 12–14 hours of light dailyIf using grow lights, keep them 5–10cm above the tops of seedlings. A bright, sunny windowsill also works well.

Step 4 — Hardening Off & Planting Out

Before going into the garden, seedlings need to be gradually acclimatised to outdoor conditions over 1–2 weeks.

1

Begin hardening off 1–2 weeks before plantingPlace pots outside in a sheltered, partially shaded spot for a few hours each day, gradually increasing exposure to sun and wind.

2

Plant out after all frost risk has passedDahlias are frost-tender. Wait until nights are reliably warm before planting into the garden.

3

Choose a full sun positionDahlias need at least 6–8 hours of direct sun per day. Choose a spot with free-draining soil and shelter from strong wind.

4

Avoid high-nitrogen fertilisersToo much nitrogen encourages leafy growth at the expense of flowers. Once established (around 3 weeks after planting), feed with a high-phosphorus fertiliser every 3–4 weeks to encourage blooms.

Step 5 — End of Season: Saving Tubers

Your seed-grown dahlias will develop tubers over the season. These can be dug, stored, and replanted the following spring — and unlike seeds, a saved tuber will reliably reproduce that exact plant.

1

After plants die back, cut stems to 5–10cm above groundWait until after the first frost has killed the foliage before digging.

2

Carefully dig tubers and brush off excess soilAvoid washing — moisture can encourage rot in storage.

3

Cure in a warm, airy spot for 1–3 daysKeep out of direct sunlight. Once cured, store in a cool, dry, frost-free location over winter in barely damp sand, vermiculite, or sawdust.
Remember: Replanting a saved tuber next season will give you an exact replica of that seedling’s flower — the only reliable way to reproduce a plant grown from seed.