For a summer-long display of cleome’s spidery blooms, the sowing schedule is tied closely to your last spring frost. Knowing exactly when to plant cleome seeds is the first step to ensuring these tall, elegant annuals thrive in your garden.
This guide provides a clear, zone-by-zone schedule and simple steps for success. You will learn the best methods for starting seeds indoors and sowing directly outside.
We will also cover how to care for your seedlings and troubleshoot common problems. By the end, you’ll have a complete plan for growing cleome from seed.
When To Plant Cleome Seeds
The ideal time to plant cleome seeds depends primarily on your local climate and the method you choose. There are two main approaches: starting seeds indoors for a head start or sowing them directly into your garden soil.
Both methods revolve around one key date: your average last spring frost. This is the date after which it is generally safe to plant tender seedlings outdoors without risk of frost damage.
You can find your estimated last frost date by searching online for your zip code or consulting your local university extension service. This date is your gardening anchor for the spring season.
Understanding Your Growing Zone
The United States Department of Agriculture (USDA) Plant Hardiness Zone Map is a crucial tool. It divides North America into zones based on average annual minimum winter temperature.
Your zone gives you a general idea of your growing season length and frost dates. While not perfect, it provides an excellent starting framework for planning.
Here is a general guideline for when to plant cleome seeds based on USDA Hardiness Zones and frost dates:
- Zones 3-4: Last frost is late May to early June. Start seeds indoors 6-8 weeks before this date, around early to mid-April. Transplant or direct sow after the frost danger has passed.
- Zones 5-6: Last frost is mid to late April. Start seeds indoors 6-8 weeks before, around late February to early March. Direct sow after the last frost date.
- Zones 7-8: Last frost is early to mid-April. You can start seeds indoors in late February. Direct sowing in early April is often very successful here.
- Zones 9-10: Frosts are rare or light. You can often direct sow cleome seeds in late fall or very early spring for blooms in the cooler months. Avoid peak summer heat for sowing.
- Zones 11+: Frost-free year-round. Plant cleome seeds during the slightly cooler, drier periods of the year for best results.
Starting Cleome Seeds Indoors
Starting seeds indoors gives your cleome a significant jump on the growing season. This is especially beneficial in regions with short summers.
You gain more control over germination conditions and protect delicate seedlings from unpredictable spring weather. The goal is to have robust, well-established plants ready to go into the ground when the soil warms.
Timing For Indoor Sowing
As a rule, sow cleome seeds indoors 6 to 8 weeks before your expected last frost date. Count back from that date on your calendar.
For example, if your last frost is around May 10th, you should plant your seeds indoors between March 15th and March 29th. This timing allows seedlings to grow strong without becoming root-bound in their containers.
Step-By-Step Indoor Seed Starting
Follow these steps for successful indoor germination and growth.
- Gather Supplies: You will need seed starting trays or small pots, a high-quality seed starting mix (not garden soil), a spray bottle for water, and clear plastic domes or plastic wrap.
- Prepare the Medium: Moisten the seed starting mix thoroughly before filling your containers. It should be damp like a wrung-out sponge, not soggy.
- Sow the Seeds: Place 2-3 seeds on the surface of the soil in each cell or pot. Cleome seeds need light to germinate, so do not cover them with soil. Gently press them into the mix to ensure good contact.
- Create Humidity: Cover the trays with a clear plastic dome or loosely with plastic wrap. This maintains consistent moisture around the seeds.
- Provide Warmth and Light: Place the trays in a warm location (70-75°F is ideal). Bottom heat from a seedling heat mat can speed germination. Once sprouts appear, immediately move them under bright grow lights or a very sunny south-facing window.
- Water Carefully: Use a spray bottle to mist the soil surface, keeping it consistently moist but not waterlogged. Avoid pouring water, which can dislodge the tiny seeds.
- Thin Seedlings: When seedlings have their first set of true leaves (not just the initial seed leaves), thin to the strongest one per pot by snipping the others at the soil line.
Direct Sowing Cleome Seeds Outdoors
Direct sowing is the simplest method and works well in many areas. Cleome develops a taproot and often prefers not to be transplanted. Sowing them where they are to grow avoids root disturbance.
This method is best for gardeners in warmer zones or for those who miss the indoor starting window. It requires patience, as plants will bloom slightly later.
Timing For Outdoor Sowing
Directly sow cleome seeds outdoors 1 to 2 weeks after your last average spring frost date. Wait until the soil has warmed up to at least 60°F.
You can test soil temperature with a simple soil thermometer. Cold, wet soil will cause seeds to rot instead of germinate. If you have a late cold snap, be prepared to cover the seeded area with a row cover.
Step-By-Step Direct Sowing Guide
- Prepare the Garden Bed: Choose a site with full sun (at least 6-8 hours daily). Work the soil to a fine tilth, removing large clumps and rocks. Incorporate some compost to improve drainage and fertility.
- Sow the Seeds: Scatter seeds lightly over the prepared soil or place them in small groups (3-4 seeds) every 12-18 inches. Remember, do not cover them; they need light.
- Ensure Soil Contact: Gently walk over the area or use the back of a rake to lightly press the seeds into the soil. This step is crucial for moisture absorption.
- Water Gently: Use a gentle shower setting on your hose nozzle or a watering can with a fine rose to moisten the area. Keep the soil consistently moist until germination, which typically takes 10-14 days.
- Thin the Seedlings: Once seedlings are a few inches tall and have several sets of leaves, thin them to the final spacing of 12-18 inches apart. This gives each plant ample room to branch and flourish.
Seasonal Considerations For Planting
While spring is the primary planting season, other times of year can work depending on your climate. Your local weather patterns are just as important as your zone.
Spring Planting
Spring is the standard and most reliable time for planting cleome seeds across most of the country. The goal is to get plants established before the intense heat of summer arrives.
Consistent moisture is key during spring germination and early growth. Watch out for late frosts and have protective covers handy just in case.
Fall Planting
In mild winter climates (Zones 8-10), you can sow cleome seeds in the fall. The plants will establish themselves during the cool, moist weather and often bloom in late winter or early spring.
Fall planting takes advantage of natural rainfall and avoids summer heat stress. It’s a great option for gardeners in the South and Southwest.
In colder zones, some gardeners experiment with “winter sowing” by scattering seeds in late fall. The seeds lay dormant and germinate naturally when conditions are right in spring. This method has variable success but can work well for hardy annuals like cleome.
Optimal Germination Conditions
Cleome seeds germinate best under specific conditions. Meeting these needs dramatically increases your success rate.
- Light: As noted, cleome seeds require light to germinate. Never bury them.
- Temperature: A consistent soil temperature of 70-75°F is ideal. Cooler soils will delay or prevent germination.
- Moisture: The seedbed must remain evenly moist. Allowing it to dry out, even once, can kill the germinating seed.
- Stratification (Optional): Some gardeners report better germination after a period of cold, moist stratification. You can mimic this by placing seeds in a damp paper towel inside a sealed plastic bag in the refrigerator for 7-10 days before sowing.
Caring For Cleome Seedlings
Whether started indoors or direct-sown, young cleome plants need attentive care to become the towering beauties they are meant to be.
Light and Water Needs
Seedlings require intense light to grow sturdy and not become leggy. Indoor seedlings need 14-16 hours under grow lights, placed just a few inches above the foliage.
Water seedlings when the top surface of the soil feels dry. Water deeply but infrequently to encourage strong root growth. Avoid overhead watering in the garden to prevent fungal diseases; use a soaker hose or water at the base of the plants.
Hardening Off Indoor Seedlings
This is a critical step that cannot be skipped. Hardening off gradually acclimates tender indoor seedlings to outdoor conditions like wind, sun, and temperature fluctuations.
- Begin 7-10 days before your planned transplant date.
- Place seedlings in a sheltered, shady spot outdoors for 1-2 hours on the first day.
- Gradually increase their time outside and their exposure to direct sunlight over the week.
- Bring them indoors at night if temperatures are forecast to drop below 50°F.
- By the end of the week, they should be able to handle full sun and stay out overnight.
Transplanting To the Garden
Transplant on a cloudy day or in the late afternoon to reduce transplant shock. Dig a hole slightly larger than the root ball.
Gently remove the seedling from its container, teasing apart any circling roots if it’s root-bound. Place it in the hole at the same depth it was growing in the pot, backfill with soil, and water thoroughly.
Troubleshooting Common Seed Starting Problems
Even with the best plans, issues can arise. Here’s how to identify and fix common problems.
Poor or No Germination
If seeds fail to sprout, the cause is usually one of three things: old seeds, incorrect temperature, or improper sowing depth.
- Always check the seed packet date. Cleome seeds lose viability after a couple years.
- Ensure the soil is warm enough. Using a heat mat for indoor starts can help.
- Double-check that seeds were not covered with soil.
Leggy, Weak Seedlings
This is almost always due to insufficient light. Seedlings stretch desperately toward a light source.
Move indoor seedlings much closer to grow lights (within 2-3 inches). For windowsill seedlings, rotate pots daily and consider supplementing with a light. Outdoors, ensure they are not shaded by taller plants.
Damping Off Disease
This fungal disease causes seedlings to suddenly collapse and die at the soil line. It is encouraged by cool, wet soil and poor air circulation.
Prevent it by using clean containers, sterile seed-starting mix, and providing good air flow. Avoid overwatering and thin seedlings promptly to reduce crowding.
Beyond Planting: Ensuring Seasonal Success
Your timing sets the stage, but ongoing care ensures a spectacular show. Cleome is relatively low-maintenance once established.
They are drought-tolerant but perform best with about an inch of water per week during dry spells. A light application of a balanced, slow-release fertilizer at planting time is usually sufficient; too much nitrogen leads to lush foliage but fewer flowers.
Cleome often self-seeds readily. If you want volunteers next year, allow some of the last seed pods to mature and drop their seeds in the fall. For a neater garden, deadhead spent flowers to encourage more blooms and prevent excessive self-sowing.
Frequently Asked Questions
Can I Plant Cleome Seeds In The Summer?
In most regions, planting cleome seeds in the middle of summer is not ideal. The intense heat and dry conditions can stress germinating seeds and young seedlings. However, in cooler northern climates with mild summers, a late spring or very early summer sowing might still produce fall blooms. It’s generally better to stick to spring or fall schedules.
How Late Can You Plant Cleome Seeds?
The latest you can plant cleome seeds depends on your first fall frost date. Cleome needs about 70-80 days from seed to flower. Count back from your first expected fall frost to ensure the plants have enough time to mature and bloom. In most areas, planting later than early July is risky for a good flower display.
Do Cleome Seeds Need Cold Stratification?
While not strictly necessary for germination, a short period of cold, moist stratification can improve and speed up germination rates for some cleome varieties, especially older seed. It’s a helpful technique if you’ve had poor results with direct sowing in the past. The refrigerator method described earlier is simple and effective.
Should I Soak Cleome Seeds Before Planting?
Soaking cleome seeds is not required and can sometimes make the tiny seeds difficult to handle. The key requirement is light and consistent surface moisture. If you choose to soak, do so for no more than a few hours, and be very careful when sowing them onto the soil surface.
Why Are My Cleome Seeds Not Germinating?
Beyond the common issues of old seed, cold soil, and deep planting, check your watering habits. Inconsistent moisture is a frequent culprit. The seedbed must stay evenly damp. A light covering of vermiculite over the seeds after sowing can help retain surface moisture while still allowing light to penetrate, which can be a useful trick in dry climates.