Zinnias are one of the easiest annuals to grow from seeds started indoors or direct sown in the garden. The colorful blooms attract pollinators like bees and beneficial insects. They also make long-lived cut flowers in bouquets and arrangements. Knowing when to plant zinnias can mean the difference between months of beautiful blooms or a short show at the end of the growing season. For the best display of flowers you need to plant zinnias at the right time. Below I’ll walk you through all the steps of when to plant zinnias.
Timing considerations for when to plant zinnias
There are various factors that influence when to plant zinnias. These include soil and air temperature, the average last frost date, the days to maturity for the varieties you wish to grow, and whether you’re direct sowing seeds outdoors or giving them a head start indoors. There are dozens of varieties of zinnias to choose from with flowers in a wide range of sizes, shapes, and colors. Some zinnias have single flowers with a single row of petals while others have semi-double or fully double blooms. Some even have flowers that look like buttons! Common blossom colors include red, pink, white, purple, yellow, orange, and green, as well as many bi-colors. When buying zinnia seeds be sure to read the information listed on the seed packet to learn about the days to maturity for that specific variety.
No matter when you plant zinnias keep in mind they grow best in a garden bed or container that receives plenty of light. Look for a site with full sun and fertile, well-drained soil. Before I direct sow seeds or transplant seedlings, I amend the planting bed with compost and an organic flower fertilizer.
3 options for when to plant zinnias
When it comes to timing, you have a few choices for when to plant zinnias.
- Early to mid-spring – Sow zinnia seeds indoors under grow lights or in a sunny window.
- late spring – Direct sow seeds or transplant seedlings into the garden.
- Late spring through early summer – Succession planting seeds or seedlings ensures months of zinnia flowers.
Below you’ll learn all about each of these planting options and methods.
Option 1 – Early to mid-spring: When to plant zinnia seeds indoors
Starting zinnia seeds indoors is an easy way to get a head start on the flowering season. That said, it’s important to sow zinnias seeds indoors at the right time. If started too early, the plants become rootbound and won’t transplant well. Zinnia seeds should be started indoors 4 to 6 weeks before the last frost date in spring.
When the time is right, gather your seed-starting supplies. Here’s what you’ll need:
- Pots or cell packs and seeding trays
- Seed-starting growing mix
- Plant labels and a waterproof marker
- Grow lights (or a sunny windowsill)
- watering can
How to sow zinnia seeds indoors
Fill pots or cell packs with the pre-moistened potting mix. Peat pots are often recommended for starting zinnia seeds because they’re said to transplant easily into the garden. However, I don’t find peat pots breaking down in my climate and if any part of the pot is above the soil, it wicks water away from the root system. For these reasons I avoid peat pots.
Plant the seeds a scant quarter inch deep and water after sowing to ensure good soil-seed contact. Move the containers to a sunny window or place them beneath a grow light. I prefer using a grow light as it produces strong, stocky seedlings. Leave the light on for 16 hours each day, using a timer to turn it on and off. Bottom water seedlings or use a watering can with a rose attachment. As the seedlings grow, feed them every 3 weeks with a fish emulsion fertilizer mixed at half strength. Harden off the seedlings about a week before you plan on moving them to the garden by slowly acclimatizing them to outdoor growing conditions.
Option 2 – Late spring: Direct sow zinnia seeds or transplant seedlings
It’s important to get the timing right when direct sowing seeds or transplanting seedlings outdoors. If you plant zinnias too early in spring, frost or cold temperatures can damage the plants. Sowing seeds in cold, wet soil can cause the seeds to rot. Let the weather be your guide. Sow seeds or transplant young plants after the last frost and when the soil temperatures have warmed to 70 F (21 C).
How to direct sow zinnia seeds
Zinnias are quick to grow from direct sown seed, with most varieties flowering about 70 days from germination. Before planting, prepare the site by amending the soil with organic matter and adding a slow release organic flower fertilizer. Zinnias are best planted in rows spaced 10 to 12 inches apart with the seeds spaced 3 inches apart. Sow the seeds to 1/4 inch deep. Keep an eye on soil moisture and aim to maintain a slightly moist soil until the seeds germinate in 7 to 10 days. When the first set of true leaves appears, thin seedlings 9 to 12 inches apart. This ensures good air circulation between the plants which can reduce the occurrence of diseases like powdery mildew.
How to transplant zinnias
As the last frost date nears, harden off indoor grown seedlings to prepare them for transplanting. If you didn’t start your own seeds indoors, you can buy packs of zinnia plants from a local garden centre. Before transplanting, prepare the garden bed by amending the soil with compost and adding a flower fertilizer. Space zinnia plants 9 to 12 inches apart, depending on the variety. Tall growing varieties should be spaced a foot apart, while more compact varieties can be spaced 9 inches apart. Leave a foot between rows.
Carefully slip the plants from their containers without disturbing the root system. Using a garden trowel, dig a hole to accommodate the size of the root ball and place the seedling in the hole. Re-fill the planting hole gently firming the soil around the roots. The seedling should be planted at the same depth it was growing in its original container. If you plant it too deeply, growth may be affected.
Option 3 – Succession plant zinnias from late spring through early summer
Zinnias are considered ‘cut and come again’ flowers because they produce blooms over a long period of time. That said, after the first month of flowering, new bud production slows and you’ll get fewer blooms. To ensure months and months of gorgeous flowers, I succession plant zinnias three times. Here’s my zinnia planting schedule which is based on my average last frost date (May 20th):
- Average last frost date (May 20th): Direct sow or transplant zinnias outdoors.
- 3 weeks later (June 10th): Direct sow seeds or transplant zinnia seedlings.
- 3 weeks later (July 1st): Direct sow seeds or transplant zinnia seedlings.
- 3 weeks later (July 22nd): My final succession planting is to start seeds indoors in late June and transplant the seedlings into the garden in late July. These plants begin to bloom by late August and continue pumping out flowers until frost.
As noted above you can succession plant by direct seeding or by transplanting seedlings. If you wish to transplant seedlings in mid-summer it will likely be hard to source young zinnia plants from local garden centres. However, it’s easy to turn your grow lights back on and start a couple pots indoors. As with early spring seed starting, sow the seeds indoors 4 to 6 weeks before you intend to move the young plants to the garden.
To watch me plant zinnia seeds and discuss more about the best timing, check out this video:
Zinnia growing tips
Whether you’re growing a cut flower garden or planting zinnias for bees and butterflies, there are a few tasks you can do to promote healthy plants and lots of beautiful blooms.
- deadheading – Succession planting is the best way to enjoy lots of gorgeous zinnia flowers, but it also helps to deadhead spent blooms regularly. Using garden snips or hand pruners, remove dead flowers several times a week. This plant directs growth to producing new flowers and not maturing seedheads.
- watering – Reduce the occurrence of diseases by spacing plants to encourage good air flow and watering properly. Zinnias are fairly drought-tolerant, but a long stretch of dry weather can slow flower production. If there has been no rain for a week, I deep water my zinnia beds. I like to use a long-handled watering wand to direct water to the roots of the plant. I try not to wet the foliage as splashing water can spread disease. A soaker hose is another easy way to water a long row of zinnia plants.
- pinching – If you’re growing zinnias as a cutting flower, I recommend pinching the young plants back. Pinching results in longer flower stems. It’s best done when the plants are 9 to 12 inches tall. Using sharp garden snips remove the top 3 to 4 inches from the plant, cutting back to a fresh set of leaves.
- pest control – Keep an eye out for zinnia pests like thrips, aphids, and spider mites. If you see holes in the leaves, or leaf spots, take a closer look. If I spot a pest like aphids, I generally knock them from the plant by spraying them with a jet of water from a hose.
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