How to Grow
Caring for ageratum and encouraging an abundance of flowers is simple and straightforward. Let’s look at the details.
Light
Floss flowers prefer a full sun location but tolerate some shade or dappled light, however flowering does decline in too much shade.
In areas with scorching hot sunshine, light afternoon shade helps to protect plants and prevent wilting.
Soil
Ageratum plants grow best in fertile, well-draining soil.
To prepare garden soil, loosen to a depth of eight to 12 inches.
For each plant, mix in a shovelful of organic material such as compostleafmold, well-rotted manure, or worm castings.
If needed, mix in one shovelful of gritty materials to improve drainage, using the likes of landscape sand, pea gravel, crushed shells, or stone chips.
Before planting, stir in one tablespoon of bone meal or a balanced, slow-release fertilizer such as 4-4-4 NPK.
If you need a recommendation, I use this all natural granular fertilizer, available at Burpee.
Use a similar fertile, well-draining potting soil for containers.
Water
For the best floral display, ageratum needs regular water applications, especially in hot weather.
Provide one inch of water per week and allow the top one or two inches of soil to dry between applications.
More frequent water applications may be needed during hot spells and for container-grown ageratum plants.
To thwart fungal pests like powdery mildew, water in the morning to manage humidity and allow plants to dry before nightfall.
Avoid overhead watering as well – a drip irrigation system is ideal.
Temperature and Humidity
As warm season annuals, floss flowers are frost tender but may also sag and wilt in overly hot temperatures.
For a profusion of flowers and foliage, moderate to warm daytime temperatures of 60 to 80°F and overnight temperatures above 50°F is the sweet zone.
Excessive humidity can also be problematic and lead to fungal issues like powdery mildew and Botrytis blight.
Fertilizer
Ageratum flowers are heavy feeders, and along with a fertile planting site, regular fertilizer applications are needed for the most prolific blooms.
After planting, scratch a slow-release balanced or flower-boosting fertilizer into the soil surface of garden beds and containers, then reapply in early summer.
Or you can use a liquid or water soluble formula and apply every three weeks.
Flower boosting formulas have a higher level of phosphorus, like this 6-12-6 (NPK) product, available at Arbico Organics.
Container plants can be fed every two or three weeks but use a half-strength solution at this frequency.
Cultivars to Select
Many attractive varieties are available in the spring as bedding plants or seed at local nurseries and online.
Here’s a sampling of some popular cultivars to start with:
Blue Horizon
A tall variety with striking, silvery mauve blooms, ‘Blue Horizon’ is ideal for cutting or in beds, containers, and cottage gardens.
The productive, robust plants grow up to 32 inches tall and 18 inches wide. Cut deep for branching stems and an abundance of flowers.
Pelleted seeds in packets of 100, 500, and 1,000 can be found at True Leaf Market.
Blue Mink
Compact and charming, ‘Blue Mink’ is a vibrant miniature variety with pale mauve flowers in a dense, mounding form.
The tussocks grow 12 inches tall and wide, adding reliable, long-lasting blooms to beds, borders, containers, and rockeries.
Seeds in packets and ounces are available at Eden Brothers.
Monarch Magic
A compact variety with vigorous growth, ‘Monarch Magic’ features masses of butterfly-attracting lavender purple flowers all summer long.
Plants grow up to 12 inches tall and 16 inches wide and generate ample flowers when cut regularly.
Bedding plants in four-packs are available at Burpee.
Red Flint
Popular for their handsome magenta flowers, ‘Red Flint’ is a mid-height cultivar that’s perfect for beds, borders, cutting, and butterfly gardens.
Plants grow up to 24 inches high and spread up to 16 inches wide.
Seeds in a variety of packet sizes are available at Eden Brothers.
Maintenance
Ageratum requires little in the way of maintenance but pinching out the buds can help plants to branch and develop more flowers.
To encourage a long, staggered flowering time, in spring after buds have formed, pinch out half the buds and leave the remainder intact.
Deadheading isn’t necessary but helps to maintain a tidy appearance.
For cut flowers, harvest a little before the buds are fully open, cutting low on the stem to promote branching and more flowers.
Cut stems above a leaf node and leave at least six inches of stem to branch.
To harvest the foliage, cut stems before flower buds form.
Propagation
Floss flower seeds can be direct sown outdoors after all danger of frost has passed.
Because the seeds are very small, direct sowing can lead to somewhat spotty results – starting them early indoors gives a better germination rate.
Here’s how to start the seeds indoors:
- Six to eight weeks before your average last frost date, sow seeds into flats filled with a moistened seed-starting mix.
- Use tweezers or the tip of a moistened wooden skewer to lift and place seeds on top of the soil then barely cover them with a thin layer of soil.
- Use a gentle touch to lightly firm the seeds in place.
- Place the trays in a warm location with bright, indirect light.
- Keep the soil lightly moist until the seedlings are ready to transplant.
- Seedlings are ready for transplanting when they have three or four sets of true leaves and overnight temperatures consistently remain above 50°F.
- Harden off for a couple of weeks before planting out into a full sun location.
- Space miniature varieties six inches apart and standard plants nine-to-12 inches apart for ample air circulation.
For a steady supply of flowers, I suggest making two or three successive plantings two or three weeks apart.
