Ageratum houstonianum (aj-eh-RA-tum hews-tun-EE-an-um) is an annual often used in garden beds or outdoor containers.
It is a member of the genus Ageratum and the family Asteraceae.
Ageratum houstonianum is native to southern Mexico, Guatemala, and Belize, though it has spread to other areas.
It grows naturally in forest clearings, pastures, and shrub-covered ground up to about 3,000′ feet.
In the United States, ageratum is a frost-tender annual, although it can be a perennial in Zones 10 and 11.
It’s often planted for its bright blue flowers, although other varieties can also be purchased in lavender, purple, red, pink, and white.
Ageratum houstonianum is also known as:
- Floss flower
- Mexican Paintbrush
- Blueweed
- Blue mink
- Pussy Foot
Quick Facts On Ageratum Houstonianum
- Family: Asteraceae
- Light: Full sun or partial shade
- Temperature: 65° to 75° degrees Fahrenheit
- Water: Moderate watering at least weekly
- Fertilizer: Granular slow-release fertilizer with high nitrogen
- Propagation: Direct broadcast of seeds
- Common Problems Aphids, whiteflies, root rot, botrytis or mildew
Ageratum Houstonianum Care
How Big Does Mexican Paintbrush Grow?
Ageratum plants can range from 4″ to 18″ inches tall. A few varieties, such as “Blue Horizon,” can grow to 30″ inches in height.
Depending on the variety, the plants can spread and cover between 6″ and 18″ inches in diameter.
When Does Mexican Paintbrush Bloom? Are The Flowers Fragrant?
Ageratums planted early in the season can flower continuously from late spring until the first frost in autumn.
However, Mexican paintbrush plants that are started from seeds planted directly in the ground might not bloom until the very end of the season, in late summer or early fall.
Blooms of the typical plant, including the varieties “Blue Mink,” “Blue Danube,” and “Hawaii,” are blue, but other types are red, pink, white, purple, or lavender.
Ageratum plants’ leaves are usually a blue-green or light green color and slightly rough.
Some varieties of ageratum houstonianum are very fragrant. All varieties are very attractive to pollinators, like bees, which flit from flower to flower for their rich nectar.
What Are The Lighting Needs And Temperature Requirements?
Ageratum prefers sun and should have direct sun if possible. They can grow in partial shade but need a longer period of sun in such cases.
For example, if you live in a place that becomes very hot in the summer, ageratum plants may do better in a location with light shade in the afternoon.
If you are growing your Mexican paintbrush plant indoors, you should put it next to a window with direct sunlight.
Then, you might move the plant back from its usual spot on the hottest days.
How And When To Water And Fertilize Mexican Paintbrush?
Ageratum should be kept moist, and watered once or twice a week. You might need to water the plants more often during the hottest part of the summer.
When planted or transplanted, the plants should be fertilized with a granular, slow-release fertilizer.
The plants might also benefit from an occasional dose of 10-10-10 fertilizer throughout the summer.
Mexican Paintbrush plants with yellow leaves lack fertilizer and will improve with applying fertilizer.
What Is The Best Soil For Ageratum And When Should You Transplant?
Ageratums are heavy feeders and need rich, well-drained soil. The soil pH should not be higher than 7.0. It is not necessary to mix in the sand when planting ageratums.
The roots are not deep, so the soil should be kept moist. The plants should be watered at the base and not sprayed on their leaves as this may cause burning.
Ageratums can be started and grown in pots, but they should only be transplanted when moving to their final home, in the garden bed, container, or box.
Does Ageratum Need Special Grooming Or Maintenance?
Ageratums don’t require deadheading. The plant grows quickly, and spent blooms fall off to be replaced by new ones.
The plants are heavy feeders and will show signs of yellow leaves if they need more nutrients.
Ageratum plants can also be invasive and spread.
The first frost will kill all or most ageratum plants, and they can be removed from the garden.
How To Propagate Ageratum?
You can start ageratum plants indoors 8 to 10 weeks before the last frost date in your area.
You can also plant seeds directly outdoors after all danger of frost has passed.
Seeds broadcast directly might not bloom until the very end of the season. In either case, push the seeds gently into the soil and do not cover them, as the plant needs light to germinate.
You should keep the soil moist until leaves emerge between 5 and 14 days after planting.
Plants purchased from a greenhouse can be transplanted into a garden bed.
Here’s how to do it:
- Pick a spot with a good sun that has rich, well-drained soil.
- Dig a hole about the size of the greenhouse container and deep enough so that the top of the rootball is level with the surface.
- Disentangle the roots if they have become pot-bound.
- Fill the soil back in, and gently press the plant into place.
In the garden bed, taller varieties of ageratum should be planted a foot apart. Shorter varieties can be planted a half-foot apart.
Ageratums do not tolerate cold well and should only be planted after all danger of frost has passed.
Floss Flower Pests And Diseases
The ageratum plant produces chemicals that are harmful to insects, so the plant has few pest problems other than occasional aphids or whiteflies.
However, it is sometimes susceptible to botrytis or mildew. In poorly drained soil, roots may rot.
Also, Floss Flowers may grow spindly and need some support. Plants needing more fertilizer may develop yellow leaves.
Is The Flossflower Considered Toxic Or Poisonous To People, Children Or Pets?
Floss flower contains pyrrolizidine alkaloids, a group of chemicals found in some plants that defend against insect pests.
Unfortunately, these chemicals are also toxic to grazing livestock and can cause liver damage.
All parts of the plant contain the toxin, so small children and pets should be kept from eating floss flowers.
Is The Flossflower Considered Invasive?
Floss flower grows readily from seed and has been declared an invasive weed in parts of the United States.
Suggested Blue Mink Uses
Ageratum, also known as blueweed or blue wink, can be used in any location where blue flowers are desired.
Blue is less common in flowers, so a blue-themed garden will almost certainly have a place for ageratum.
Ageratum is also often used in cottage garden plans, as the puff-shaped flowers can lend a quaint feeling to the planting.
Taller varieties can be used when a vertical component is important. For example, the height of the plant can be enhanced by planting it in a container or box.
Shorter varieties can be planted in hanging baskets or other containers or used in beds or along the edge of walkways.
They can also be used in rock gardens.