How to Plant, Grow, and Care for Ageratum

If you’re looking for a flower that serves a supporting role in the garden and floral studio, ageratum is one of your best choices. These easy-to-care-for annuals produce clusters of blooms topped with tufts of silk-like ray flowers that give a unique and soft look.

While they won’t steal the spotlight in the garden, they add a soft texture and subtle pop of color. Most of the flowers are blue, but you can also find varieties in shades of pink, purple, and white.

The plants continue to pump out new blooms for multiple months, providing long-lasting color. Plus, the blooms hold up well when cut and attract pollinators of all kinds.

What Is It?

Ageratum goes by several names, including floss flower, blue mist flower, and bluemink. It’s an annual plant that’s native to Central America but widely grown throughout the world as an ornamental. In some locations, it’s escaped cultivation and become a nuisance plant.

You can find multiple varieties of Ageratum houstonianum that vary in height and flower color. Some of these cultivars work well as short bedding plants, and others produce long stems that make them suitable options for filler flowers in arrangements.

Characteristics

Ageratum is a flowering annual that grows best in summer. It has a mounding growth habit with densely packed, branching stems covered with slightly fuzzy, ovate leaves. Some varieties rarely grow more than six inches tall, while others send out two-foot-long stems.

Clusters of small, round flowers appear on the top of the stems. Blue is the most common flower color, but you can also find varieties with white, purple, or pink blooms. All of the flowers have elongated ray florets that look like tufts of silky floss.

Some people confuse ageratum with blue mistflower (Conoclinium coelestinum). While both of these plants produce similar flowers, the latter is a cold-tolerant perennial that’s native to the eastern half of the United States. 

Native Range

Ageratum is native to portions of Central America and Mexico. Although it can complete its life cycle in a few months, it often remains living for multiple years in these frost-free areas.

Planting

Whether you want to tuck a few ageratum plants into a planter or front yard garden bed, or add taller varieties to your cutting garden, starting with healthy plants is key. Since ageratum is an annual, you’ll need to replant these flowers each year. Starting with seeds or transplants are both viable options, so choose the one that works best for you.

Starting From Seed

If you want to grow unique ageratum varieties or plan to add dozens of these plants to your garden, starting from seed is the best option. This method gives you full control over the growing process—you can grow the number of seedlings you want and ensure they’re ready to plant at the ideal time.

Since ageratum isn’t frost-tolerant, you should wait to move your seedlings outdoors after your last spring frost. Therefore, avoid starting seeds too early. Sowing seeds indoors six to eight weeks before your last frost will give you seedlings that are ready to transplant at the proper time.

Begin by filling seed-starting trays with a well-draining soil mix designed for seed starting. Alternatively, you can use soil blocks. Regardless of which method you choose, place one to two seeds per cell or block.

Ageratum seeds are teeny tiny, so they can be difficult to handle. If your fingers aren’t well-suited for the task of sorting the tiny seeds, use a moistened toothpick to lift the seeds into the trays. Ageratum seeds require light to germinate, so leave them uncovered.

After you’ve finished seeding, water the seeds thoroughly. Bottom watering is ideal since it avoids disrupting or covering the tiny seeds. Just place your cell trays on a bottom tray, add a half-inch or so of water, wait for the soil mix to drink up the water, then remove the excess water from the bottom tray. You can also use a spray bottle to gently mist the tops of the seed trays.

Set the seeds somewhere between 70-80°F (21-29°C), and ensure they are not in total darkness. They don’t need to be under grow lights, but avoid placing them in a dark closet or basement.

If you keep the soil warm and moist, you should see seeds begin to germinate in one to two weeks. Move the sprouted seeds to an area that receives at least ten hours of bright light. You can use a grow light to provide supplemental lighting and support the seedlings as they grow.

Transplanting

Whether you grow your own seedlings or purchase some from a nursery, you’ll need to transplant them into your garden. The first step is waiting until the last frost has passed. Ideally, you’ll wait until the nighttime temperatures are steady at 40°F (4°C) and above.

The ideal spacing for ageratum depends on the variety, so check your seed packet or plant label for information. Next, find a location with at least eight hours of light and dig a hole the size of the plant’s root ball. Place the seedling in the hole, then water it thoroughly.

How to Grow

Ageratum is pretty easy to grow, as long as you provide the right growing conditions. Keep these requirements in mind when planting and caring for this cheerful annual.

Light

Full sun is ideal for ageratum as it provides the plants with the energy they need to grow lush foliage and produce lots of their cheerful blooms. Aim to place the plants in a location that receives at least eight hours of sun each day. The plants can tolerate a few hours of morning or afternoon shade, but avoid planting them in full shade.

Water

Ageratum can tolerate light drought, but it will remain happiest if you keep the soil consistently moist throughout its life. Young seedlings have smaller root systems than mature plants, so plan to water these small plants more often. One ageratum is well established, you can get away with watering the plants deeply one or two times per week, especially if the soil is mulched.

Drip irrigation is preferred over overhead sprinklers since it keeps the foliage dry and delivers water directly to the soil. You can also use a hose or a watering can to deliver water near the base of the plants.

Since potting mix often dries quicker than native soil, plan to water container-grown plants more often. If your potted plants are in full sun, you may need to water every day during the summer.

Soil

This annual grows best in loose, well-drained soil that’s rich in organic matter. However, it can grow in soils high in clay or sand as long as it’s well-draining and watered appropriately. If you notice you’re dealing with compacted soil, use a digging fork to loosen the soil before planting.

If you’re planting ageratum in a container, choose a well-draining potting mix rich in organic matter. You want the mix to allow excess water to escape while also retaining moisture between irrigation events.

Temperature and Humidity

Ageratum is a warm-season annual that thrives when temperatures remain between 55-85°F (13-29°C). It can tolerate temperatures outside of this range, but high heat and cold will stress the plants. The plants are not frost-tolerant and will die when the temperature drops below 32°F (0°C).

The plants can tolerate a range of humidity levels, but they are more susceptible to fungal diseases when humidity is high. If you live in a humid area, ensure adequate plant spacing to allow for good airflow.

Fertilizing

Ageratum isn’t a heavy feeder, so applying nutrient-rich compost at planting time will often carry the plants throughout the growing season. However, if you’re starting with poor soil, you may want to add additional nutrients. Extra compost or a fertilizer designed for flowering plants will provide ageratum with the nutrients it needs.

If you apply fertilizer, make sure to stay away from high-nitrogen products. Adding too much nitrogen will encourage the plants to produce lush foliage but few flowers. Instead, look for a balanced fertilizer with equal amounts of nitrogen, phosphorus, and potassium.

Maintenance

This isn’t a needy plant, so you won’t have to commit lots of time to keep it looking good. The main maintenance involves removing spent flowers to encourage the plants to produce new blooms. You can use a pair of clippers to remove any flowers that have turned brown.

Pinching young seedlings is optional, but it encourages the plants to develop a bushier growth habit. When the seeds have three to four sets of true leaves, remove the top bud. This will encourage the side shoots to grow longer, leading to a larger plant and more flowers.

Common Issues

Ageratum rarely develops serious pest and disease issues, especially compared to other cut flowers. However, be aware of the following issues and address them as necessary.

Powdery Mildew

This fungal disease coats ageratum leaves and flowers in a dusting of fine, white powder. Although the disease doesn’t mean your plant is at risk of imminent death, it can negatively impact the plant’s health over time.

Unlike many fungal diseases, powdery mildew is most likely to occur during warm and dry yet humid conditions. Therefore, you’ll often see it pop up during dry spells in the summer.

Preventing powdery mildew is often easier than treating the disease, so aim to keep your plants well-watered. Avoid overhead watering and provide adequate plant spacing to allow for good airflow.

If your plants develop powdery mildew, you don’t necessarily have to treat the fungus. Minor cases rarely kill the plants and only make them less visually appealing. If you choose to treat powdery mildew, you can use organic fungicides.

Aphids

Aphids are small, sap-sucking insects that attack many plants, including ageratum. Although a few of the pests aren’t a problem, these insects can quickly multiply to form large, damaging populations. Plants can easily become weakened and discolored when hundreds of aphids are drinking their sap.

One way to prevent aphid outbreaks is to maintain healthy natural predator populations. Planting a diversity of flowering plants and avoiding broad-spectrum insecticides will encourage healthy numbers of ladybugs, lacewings, parasitic wasps, and other beneficials. Although these predators won’t stop aphids entirely, they will prevent their numbers from getting out of control.

Regularly inspecting your plants for aphids is another way to prevent them from becoming a big problem. If you spot a few of the pests on your ageratum, you can squish them with your fingers or spray them with a strong blast of water. Larger populations may require that you spray them with neem oil or insecticidal soap.

Frequently Asked Questions