Plant ageratum after frost, or start seed 8 to 10 weeks indoors. Set the crown at nursery depth, and space compact plants 6 to 8 inches.
Plant after frost, or start seed 8 to 10 weeks indoors
| Factor | Recommended | Avoid |
|---|---|---|
| When — trays | After the last frost, once nights stay mild | Setting tender plants out into a late freeze |
| When — seed | Start 8 to 10 weeks before last frost; or sow outdoors after frost | Burying seed, or sowing into cold soil |
| Depth | Crown at nursery-pot level; seed barely covered or pressed on the surface | Planting deep, or covering seed so light cannot reach it |
| Spacing | Compact faces 6 to 8 inches; taller about 12 inches | A packed ribbon with no air between leaves |
Wait until frost is done. Ageratum is a heat-loving annual and stalls in cold nights. [1] Start seed indoors 8 to 10 weeks before your last frost if you want earlier bloom. [2] You can sow outdoors after frost, but bloom arrives late. Harden trays before they go in the bed. [3]
When should I plant ageratum outdoors?
After all danger of frost, when nights stay mild. Harden off transplants first.
How to plant ageratum
Work one patch at a time: prepare moist fertile ground that drains, set the plant, then water to settle.
- Confirm Ageratum houstonianum — not hardy ageratum (Conoclinium).
- For seed: surface-sow or barely cover. Light helps germination. Keep warm and moist for about 1 to 3 weeks, then prick out when seedlings handle.
- Dig a hole the depth of the nursery pot. Set the crown level with the soil surface. Backfill and firm.
- Water at plant-out. Keep the top evenly moist while roots take. Space compact plants 6 to 8 inches; give taller cut faces about 12 inches.
How do you start ageratum from seed?
Start indoors 8 to 10 weeks before last frost. Surface sow or barely cover — light aids germination. Transplant after frost.
Space compact plants 6 to 8 inches — and what fails
Compact mounds need 6 to 8 inches so leaves dry between waterings. Taller cut stems want about 12 inches. [4] Crowding plus overhead water is the usual leaf-spot setup.
How far apart do I plant?
Compact faces 6 to 8 inches. Taller cut stems about 12 inches so leaves can dry.
- Planted before frost — seedlings stall or blacken.
- Seed buried — light never reaches it, so it never comes up.
- Crown sunk below grade — stem rot in wet spells.
- Packed tighter than 6 inches — mildew in humid weather.
If seedlings stall after a cold night, wait for mild weather and plant the next tray then.
FAQ
Can I sow outdoors after frost?
Yes, but bloom runs late. Indoor start 8 to 10 weeks before frost gives a longer season.
Why did the plants fail?
Usually a late frost, seed covered too deep, or a crown planted below grade in wet soil. Reset the next tray after frost.
Related guides
Recommended plants
References
- [1] Garden Design -
https://www.gardendesign.com/annuals/ageratum.html - [2] Wisconsin Horticulture -
https://hort.extension.wisc.edu/articles/ageratum/ - [3] Illinois Extension -
https://extension.illinois.edu/flowers/ageratum - [4] NC State Extension Gardener -
https://plants.ces.ncsu.edu/plants/ageratum-houstonianum/
This note is part of Ageratum Guides.
More planting notes by plant are gathered in Planting Guides.
Updated: Reviewed by Bosquelle Editors