When To Plant Cleome Seeds

Knowing when to plant cleome seeds is the single most important factor for getting a stunning, long-lasting display of these unique flowers. If you get the timing right, these easy-care annuals will reward you with towers of intricate, spider-like blooms from early summer straight through to frost. This guide will give you the clear, simple steps to nail the timing in your specific garden.

Cleomes, often called spider flowers, are a classic cottage garden favorite. They bring height, airy texture, and a wonderful range of pinks, purples, and whites to your beds. Best of all, they’re incredibly low-maintenance once established. But it all starts with sowing the seed at the correct moment.

There are two primary windows for planting cleome seeds: in late spring directly outdoors, or indoors several weeks before your last frost. The best method depends on your climate and how patient you are for those first blooms.

Understanding Your Frost Dates

Your local average last spring frost date is your gardening anchor. This date tells you when it’s generally safe to plant tender seeds and seedlings outside. You can find yours with a quick online search or by asking at a local nursery.

For Direct Sowing Outdoors: Plant seeds 1-2 weeks after your last frost date, when the soil has warmed up.
For Starting Seeds Indoors: Begin 6-8 weeks before your last frost date.

Option 1: Direct Sowing Cleome Seeds

This is the easiest method and works well in most regions. Cleomes have a good germination rate when sown directly in their final growing spot.

Best Time: Wait until all danger of frost has passed and the soil is no longer cold and soggy. A good rule is when daytime temperatures are consistently above 60°F (15°C). This is usually late spring.

How to Direct Sow:

Choose a sunny location. Cleomes need at least 6-8 hours of direct sun.
Loosen the soil and rake it smooth. Cleomes aren’t picky about soil but good drainage helps.
Scatter the seeds lightly on the soil surface or press them in gently. They need light to germinate, so do not cover them with soil.
Water gently with a fine mist to settle the seeds.
Keep the area moist until seedlings appear, which can take 10-14 days.

Option 2: Starting Cleome Seeds Indoors

Starting seeds indoors gives you a head start, especially in areas with shorter growing seasons. You’ll see blooms several weeks earlier than with direct-sown plants.

Best Time: Count back 6-8 weeks from your expected last frost date. For many, this is in early to mid-spring.

How to Start Seeds Indoors:

Fill seed trays or pots with a sterile seed-starting mix.
Press the seeds lightly onto the surface of the moist soil. Do not bury them.
Place the trays in a warm location (70-75°F is ideal) with bright light. A sunny windowsill or under grow lights works well.
Keep the soil consistently moist but not waterlogged. A clear plastic dome can help retain humidity until seeds sprout.
Once seedlings have a few sets of true leaves, you can thin or transplant them into individual pots.

Hardening Off and Transplanting

If you start seeds indoors, you must “harden off” the seedlings before planting them outside. This process acclimates them to sun, wind, and cooler temperatures.

Begin 7-10 days before your planned transplant date.
Place seedlings outdoors in a sheltered, shady spot for a few hours each day, gradually increasing their time outside and exposure to sun.
After a week, they should be ready to handle full sun and wind.
Transplant them into the garden on a cloudy day or in the evening to reduce shock.

Regional Timing Guide

Your location greatly influences the perfect planting window. Here’s a general breakdown:

Northern Climates (Short Growing Season): Start seeds indoors 6-8 weeks before the last frost (often late April to May). Direct sowing is possible but blooming will start later.
Midwest & Mid-Atlantic: A mix works well. Start indoors in early April for early blooms, or direct sow in mid-to-late May.
Southern Climates (Long Growing Season): You can direct sow in early spring after the last frost. You can also plant in late summer for a fall bloom show.
Mild Winter Climates (West Coast, Gulf Coast): Cleomes can often be sown in fall for winter or very early spring blooms, as they tolerate mild frosts once established.

Tips for Optimal Germination

Cleome seeds can be stubborn sometimes. These tricks can improve your success rate significantly.

Light Exposure: Never cover the seeds. Light is crucial for germination.
Pre-Chilling (Stratification): Some gardeners find that placing seeds in the refrigerator for a week before sowing helps mimic winter and improve germination.
Soaking Seeds: Soaking cleome seeds in lukewarm water for 12-24 hours before planting can soften the seed coat and speed up sprouting.
Patience is Key: Germination can be irregular. Don’t give up on a pot or area for at least 3 weeks.

Caring for Your Young Cleome Plants

Once your seeds have sprouted, a little care ensures strong, blooming plants.

Thinning: If direct sown, thin seedlings to stand 12-18 inches apart once they are a few inches tall. This gives them room to grow into their full, bushy size.
Watering: Water young plants regularly until they are established. Mature cleomes are very drought-tolerant.
Pinching: When plants are about 12 inches tall, you can pinch off the top few inches. This encourages bushier growth and more flower stems.
Fertilizer: Cleomes don’t need rich soil. A light application of a balanced fertilizer at planting time is usually plenty. Too much fertilizer can lead to lots of leaves and fewer flowers.

Common Mistakes to Avoid

Even experienced gardeners can make a few errors with cleomes. Here’s what to watch out for.

Planting Too Early: Cold, wet soil will cause seeds to rot. Wait for warm soil.
Burying the Seeds: This is the most common reason for poor germination. Just press them in; don’t cover them.
Overcrowding: It’s tempting to leave all the seedlings, but crowded plants will be weak and prone to disease. Thin them bravely.
Overwatering Mature Plants: Once established, cleomes prefer drier conditions. Overwatering can cause root problems.

Encouraging Blooms and Self-Seeding

Cleomes are fantastic self-seeders, often giving you free plants year after year.

To encourage self-seeding, allow some of the last flowers of the season to form long, slender seed pods and drop their seeds.
In spring, you’ll see volunteer seedlings. You can thin and move these volunteers to where you want them.
Deadheading (removing spent blooms) can promote more flowering, but leaving some pods to mature is how you ensure future generations.

FAQ: Your Cleome Seed Questions Answered

Can I just scatter cleome seeds?

Yes, scattering is a valid method for direct sowing! The key is to do it at the right time (after frost) and in a sunny spot. Rake the area lightly after scattering to ensure good seed-to-soil contact, but don’t cover them deeply. You will need to thin the seedlings later.

How late can you plant cleome seeds?

For a first-year bloom, you should plant seeds no later than early summer. They need enough time to grow and flower before fall frost. In very warm climates, you might get away with a late summer planting for fall color. If you miss the window, you can always sow seeds in late fall to overwinter and sprout in spring.

Do cleome seeds need cold stratification?

It’s not strictly necessary, but it can help. Cleome seeds have a hard coat. A period of cold, moist conditions (like a week in the fridge) can mimic nature and break dormancy for more even germination, especially for older seeds.

Why didn’t my cleome seeds come up?

Several factors could be at play. The most likely are: planting too deep (they need light), soil was too cold or wet, seeds were old and lost viability, or the soil dried out during the critical germination period. Always check your seed packet for a “packed for” date and store extra seeds in a cool, dry place.

What is the best month to plant cleome?

The best month depends entirely on your zone. In most of the US, it’s typically May for direct sowing outdoors. For indoor starting, March or April is common. In the Deep South, February or March might be your outdoor window, while in cooler Northwest areas, you might wait until June.

Getting the timing right for when to plant cleome seeds sets the stage for a truly spectacular garden performance. With their tall, graceful stems and constant production of blooms, they fill a niche that few other annuals can. By following these simple guidelines—waiting for warm soil, giving them light, and providing basic care—you’ll be rewarded with a beautiful, low-fuss display that lasts for months. And with their self-seeding habit, you might just find that these spider flowers become a beloved, permanent fixture in your garden landscape.