Ageratum in Zone 7A — Mountain West

Ageratum, with its unique fuzzy blue-purple blooms, is a fantastic addition to any Mountain West garden. It thrives in our intense sunlight and the cool nights even in summer enhance its vibrant color. Plus, it's a magnet for pollinators, adding life and beneficial insects to your garden.

Don't let our short season intimidate you. Starting seeds indoors gives you a head start, and Ageratum matures quickly. With a 173-day growing season, you have plenty of time to enjoy these charming flowers, as long as you plan accordingly for our alkaline soil, intense UV, and possible hail.

Starting Ageratum indoors is a good idea in our region, especially if you want earlier blooms. Begin in early to late March, about 8 weeks before you plan to transplant outdoors. This gives the plants a jump start before our late spring arrives.

Use seed trays with a good seed-starting mix, and provide warmth and plenty of light. A heat mat can help with germination. Remember to water from the bottom to prevent damping off and encourage strong root growth.

Keep in mind that our spring weather can be unpredictable. Even with a head start indoors, be prepared to protect your seedlings from late frosts when you eventually move them outside.

Transplant your Ageratum seedlings outdoors from early May through early June, after the danger of frost has passed. Before transplanting, harden off your seedlings by gradually exposing them to outdoor conditions for about a week. This will help them adjust to our intense sun and cooler temperatures.

Plant them 6-8 inches apart to allow for good air circulation. Be mindful of potential late frosts or unexpected temperature dips that are common in our Mountain climate during this time. Row covers can provide extra protection if needed.

Ageratum needs moderate watering, so aim to keep the soil consistently moist, but not soggy. In our dry Mountain West climate, that can be a bit of a balancing act.

During the hottest part of the summer, you'll likely need to water deeply a couple of times a week. Use the "finger test" – stick your finger about 2 inches into the soil. If it feels dry, it's time to water. Aim for about 1 inch of water per week, either from rainfall or irrigation.

Water at the base of the plants to avoid wetting the foliage, which can help prevent fungal diseases in our low-humidity environment. Signs of underwatering include wilting leaves, while yellowing leaves can indicate overwatering. A light layer of mulch around the plants can help retain moisture and regulate soil temperature, which is especially useful given our intense sun and cool nights.

Deadheading is key to keeping your Ageratum blooming throughout the season. Simply pinch or cut off the spent flower heads as they fade. This encourages the plant to produce more flowers instead of setting seed.

Continue deadheading regularly throughout the summer and early fall. As the first frost approaches in early October, you can stop deadheading and let the plants set seed if you want to collect them for next year. Otherwise, remove the plants after a hard freeze.

While you don't "harvest" Ageratum in the traditional sense, you can enjoy its blooms from early July through late August. These fuzzy purple-blue flower clusters will brighten up your garden all summer long, and they are fantastic for attracting pollinators.

To enjoy continuous blooms, keep deadheading the spent flowers. This encourages the plant to keep producing new flowers. When cutting the flowers, use clean, sharp pruners to avoid damaging the plant.

As the first frost approaches in early October, let the last blooms remain on the plant to mature and set seed if you wish. Otherwise, remove the plants after the first hard freeze.

Here are the most common problems you might encounter with Ageratum in the Mountain West:

Whiteflies

• What it looks like: Tiny, white, flying insects that cluster on the undersides of leaves. Leaves may turn yellow and drop.
• What causes it: These pests thrive in warm, dry conditions, which we often experience during our summers.
• How to fix/prevent it: Regularly inspect plants. Blast them off with a strong stream of water. Insecticidal soap or neem oil can also be effective. Encourage beneficial insects like ladybugs and lacewings.

Powdery Mildew

• What it looks like: White or grayish powdery coating on leaves, stems, and sometimes fruit. Leaves curl, yellow, and die. Reduces plant vigor.
• What causes it: Various fungi that thrive in warm days, cool nights, and moderate humidity. Dry leaf surfaces actually favor it (unlike most fungi).
• How to fix/prevent it: Improve air circulation through spacing and pruning. Remove affected leaves. Neem oil or potassium bicarbonate sprays help. Milk spray (1 part milk to 9 parts water) is surprisingly effective. Plant resistant varieties.

Crown Rot in Wet Soil

• What it looks like: Stem near the soil line becomes soft, mushy, and brown or black. Plant wilts and dies.
• What causes it: Fungal disease that thrives in overly wet conditions. While our region is generally dry, overwatering or poorly draining soil can create the perfect environment for this disease.
• How to fix/prevent it: Ensure good soil drainage. Avoid overwatering. Remove and discard affected plants. Improve air circulation around the base of the plants.

Mountain West Specific Challenges: Our moderate-at-altitude heat, low humidity, and dry conditions can actually be beneficial for Ageratum, as they reduce the risk of many fungal diseases. However, the intense sun can be harsh on young plants, so be sure to provide some afternoon shade, especially during the hottest part of the summer. Also, be vigilant about watering, as our dry climate can quickly lead to drought stress.

Ageratum makes excellent companion plants in your Mountain West garden. Plant them near tomatoes and peppers. Ageratum attracts beneficial insects like hoverflies and ladybugs, which prey on common tomato and pepper pests.

Green beans also benefit from Ageratum's presence. The pollinators attracted to the Ageratum flowers help increase bean production. I'd avoid planting Ageratum near plants that prefer drier soil, as Ageratum likes moderate moisture.