Blue flowers for the garden are sometimes difficult to grow. Choices are limited and most require a full sun location. Ageratum plants, with fluffy blue flowers, add the desirable blue color to your garden, even if it is partially shaded. Caring for ageratums is simple and easy, particularly for the beginning gardener. The ageratum flower most commonly found in the garden is a hybrid, growing in a petite and compact form. When you learn how to plant ageratum and grow it successfully, you will always have a blue flower option for the bed or border.
I. Appearance and Characteristics
Ageratum houstonianum, commonly known as flossflower, bluemink, blueweed, pussy foot or Mexican paintbrush, is a cool-season annual plant often grown as bedding in gardens.
This herbaceous annual or dwarf shrub grows to 0.3–1 m (1 ft 0 in – 3 ft 3 in) high, with ovate to triangular leaves 2–7 cm (0.79–2.76 in) long, and blue flowerheads (sometimes white, pink, or purple). The flower heads are borne in dense combs. The ray flowers are threadlike and fluff-haired, leading to the common name. The narrow lanceolate bracts are pointed, denticulate only at the top and glandular hairy. The flowering period is from May to November in the northern hemisphere. The plant attracts butterflies.
Ageratum has evolved a unique method of protecting itself from insects: it produces a methoprene-like compound which interferes with the normal function of the corpus allatum, the organ responsible for secreting juvenile hormone during insect growth and development. This chemical triggers the next molting cycle to prematurely develop adult structures, and can render most insects sterile if ingested in large enough quantities.
The plant is native to Central America in Guatemala and Belize, and adjacent parts of Mexico, but has become an invasive weed in other areas. It was also naturalized in large parts of the tropics and in the southern United States. Its habitat is pastures, moist forest clearings and bushes up to altitudes of 1,000 meters (3,300 ft).
Today, it is widely used as an ornamental plant for summer borders and balcony boxes, high varieties also as cut flowers. The species is cultivated once a year, having numerous varieties whose crowns may be dark blue, purple, pink and white. Preferring cool soils and exposure in full sun, high varieties reach stature heights up to 60 centimeters (24 in).
Ageratum houstonianum is toxic to grazing animals, causing liver lesions. It contains pyrrolizidine alkaloids.
Ageratum houstonianum is prone to becoming a rampant environmental weed when grown outside of its natural range. It has become an invasive weed in the United States, Australia, Europe, Africa, China, Japan, New Zealand, and the Philippines.
II. How to Grow and Care
Sunlight
Ageratum can be grown in full sun or part shade, but keep in mind that with less sun exposure, the bloom might be less prolific, and the plant might become less compact and leggier.
Soil and Water
Ageratum should be planted in fertile soil with good drainage. Water the plants regularly until they are established. The plants need about 1 inch of water per week so if it does not rain, you will need to water them. Container plants require more frequent watering—daily in hot summer weather.
Temperature and Humidity
Ageratums are warm-season annuals; cold is their enemy. If they have been planted too early in the spring and then a cold spell hits, they can get easily wiped out. If you get early frosts at the end of the summer, you can extend their lifespan by covering the plants.
Fertilizing
Their extended bloom makes ageratums heavy feeders. Fertilize in-ground or potted plants about once a month throughout the growing season with a general-purpose fertilizer. The plants will usually let you know when they need more food—they are quick to develop yellow leaves when they lack nutrients.
Planting Instructions
Plant ageratum after the last frost date. The young plants should be hardened off before planting them outside.
Ageratum roots are shallow, so you don’t need to dig a hole much deeper than the pot the seedling comes in. Space them 9 to 12 inches apart. Planting them too close together will lead to poor air circulation and can encourage the spread of disease.
Pruning
Ageratums are so floriferous and fast-growing that there is no need to deadhead spent blooms. The plant will quickly overgrow them, which is one of the reasons it is considered a low-maintenance annual.
Propagation
Garden centers sell these tough little plants in multi-packs in the spring, but you can also grow them from seed. Start them indoors 4 to 6 weeks before the first frost-free day. Make sure not to cover the seeds with soil because they need light to germinate. As soon as the last frost date has passed, plant them outside.
Potting and Repotting
With their compact size, the plants also make excellent container plants. Use a well-draining potting soil and feed them with a slow-release fertilizer.
Pests and Diseases
When cared for properly, the ageratum flower is generally hardy. However, you need to look out for a few pests.
Several fungal and bacterial infections target Ageratum houstonianum. The good news is – that these infections are usually treatable.
Powdery mildew, Botrytis, and root rot will attack when soil moisture or humidity gets too high.
Insect infestations may include:
- Aphids
- Fungus gnats
- Plant lice
- Red spider mites
- Thrips
- Whiteflies can weaken the plant and cause the pointy leaves to yellow
Lure away whiteflies using an old-fashioned fly trap. Make one by painting a post bright yellow and covering it with a sugar solution, honey, or any other sticky substance.
Your Ageratum probably succumbed to a bacterial infection if you discover:
- Severe wilting, yellowing, or dying Agregatum leaves
- Find any lesions on the stems
You need to remove the unhealthy plant to ensure that the infection does not spread.
You may find mold on the flowers and stems or brown spots on the foliage of your plants. They will probably survive the problem through the season.
It is advisable to thin your plants to allow more airflow and air circulation around the plants.
Sunlight will keep:
- The flowers and leaves drier
- Prevent molds from damaging plants more
Generally, Ageratums are rabbit and deer tolerant. Deer may munch on the leaves when there’s a shortage of other food resources.
Be warned that Ageratum houstonianum contains pyrrolizidine alkaloids. Grazing animals may develop liver lesions from consuming this plant.
The plant’s ability to spread has given it a reputation in much of the world as an invasive weed. Seeds can travel not only by wind or water, but by fur, clothing, vehicles, and produce.
While naturalized throughout much of North America, it is considered invasive in Hawaii.
III. Types of Ageratum
- Artist Purple Ageratum
Rich purple blooms cover these small mounding plants with good heat tolerance.
- ‘Blue Danube’ Ageratum
Ageratum ‘Blue Danube’ bears lavender-blue flowers and grows only 8 inches tall.
‘Hawaii White’ Ageratum
Ageratum ‘Hawaii White’ grows 6-8 inches tall and has white flowers.
IV. Uses and Benefits
One of the best qualities of this popular ground cover is its ability to repel mosquitoes while attracting a range of pollinators and butterflies. It’s known to be a key nectar source for the vulnerable Eumaeus atala butterfly of Florida.
This annual flower is great for a wide range of displays, from edging borders and rock gardens to window flower boxes, pots, and as a bedding plant.
Its natural, long-lasting blue flowers of the Houstonianum Ageratum draw attention in flower beds, while the color variants and pleasant smell give a reason to pause during a long day. The fragrance also makes these great cut flowers for table displays.
Another interesting value of the plant is its ability to reduce the presence of certain nematodes and other infectious pests while attracting beneficial insects.
When used in complementary farming, it has displayed the ability to reduce fungal infections in tomato plants. Some evidence also suggests essential oils may help repel some tick species.
It is one of the few annual plants tolerant of light shade. Ageratum blooms from late spring through late summer and all the above qualities make this species of flowering plant a fine choice for outdoor containers.
