Zinnias are among the most generous summer flowers we can grow. They come in many lovely colors, are productive and fantastic as cut flowers. At the same time, they are heat-loving and sensitive to cold, which means that growing them in the Nordic countries requires a little more thought than in warmer climates.
Here I share how I grow zinnias in Sweden, based on my own experience and inspiration from other very experienced growers.
⚠️ Important to know before you start
Zinnias are extremely frost sensitive.
👉 Never plant zinnias until the risk of frost is completely over and the soil has warmed up properly.
General growing advice
Sowing and germination
- Zinnia seeds usually germinate in 5-15 days.
- Optimal germination occurs at approximately 20–24 °C.
- Sow 1–2 seeds per pot and thin as needed.
Pinching
Zinnias are pinched to make them bushier and produce more flowers. Top when the plant: is about 15–20 cm tall, or has developed 3–4 pairs of characteristic leaves.
Cut off the main shoot just above a pair of leaves.
Watering – less is often more
Zinnias prefer regular but sparing watering. They prefer to dry out slightly between waterings rather than remain wet.
👉 Water thoroughly, let dry slightly, repeat.
Nutrients
Zinnias are not particularly demanding on nutrients. A basic fertilizer soil is often enough for the entire season. Too much nitrogen produces a lot of leaves – but fewer flowers.
Flowering – when can you expect flowers?
Under good conditions, zinnias bloom about 75–85 days after germination.
⚠️ This requires:
- proper heat
- lots of sun
- no cold stress
In cool summers or cold soil, flowering may be delayed.
Special advice for our Nordic climate
Pre-cultivation – almost always preferable. In the Nordic countries the season is short. Pre-cultivation provides:
- earlier flowering
- stronger seedlings
- better use of the summer
Sow indoors 4–6 weeks before planting out, but not earlier – zinnias don't like to be left in pots for too long.
Transplantation
- Wait until night temperatures are stable above +8–10 °C
- The earth should be warm, not just the air
- Harden off the plants carefully
When direct sowing: ensure that the risk of frost is completely over and that the soil is warm. The seeds can germinate quickly, but cold periods after germination will slow down development considerably.
Greenhouse and growing tunnel – strongly recommended.
If you have the opportunity, this is the best way to grow zinnias in the Nordic countries.
Greenhouses or tunnels with open sides for good air circulation provide protection from cold nights, which contributes to faster growth and earlier flowering.
⚠️ Stagnant, humid air increases the risk of mildew and mold - ventilation is crucial.
Common mistakes
❌ Start them indoors too early. Growth stops once they are planted outside. Zinnia has been in a cramped pot for too long.
❌ Plant out too early
❌ Too dense planting without air
❌ Too much fertilizer
Summary
Zinnias are great to grow in the Nordic countries – but they must be treated as heat-loving summer flowers.
The key to success is:
- right timing
- warm earth
- sparing watering
- good air circulation
- Patience
When conditions are right, zinnias reward you with abundant flowering all summer long until frost.
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