Yarrow in Zone 7A — Mountain West

Yarrow is a fantastic addition to any Mountain West garden. Its feathery foliage and colorful blooms bring beauty and attract beneficial insects, which is a boon for our short growing season. Plus, the cool nights even in summer concentrate the flavors and scents, making it a joy to grow.

Our high altitude, intense sun, and alkaline soil can pose challenges, but Yarrow is surprisingly well-suited to our climate. With a little planning, you can enjoy a long harvest from your Yarrow patch within our roughly 173-day growing season.

Starting Yarrow indoors isn't strictly necessary, but it can give you a head start, especially if you want blooms the first year. Begin in early to late February, about 8 weeks before you plan to transplant. Use seed trays filled with a well-draining seed-starting mix.

Provide warmth and plenty of light – a grow light is helpful, as our late spring can mean less natural light indoors. Bottom watering is a great way to keep the soil consistently moist without damping off. Remember, our spring often has temperature swings, so indoor starting provides more control.

Transplant your Yarrow seedlings outdoors in early April through early May, once the danger of hard frost has passed. Before transplanting, harden them off by gradually exposing them to outdoor conditions for about a week. This is critical in our region, where intense sun and wind can quickly stress tender seedlings.

Choose a sunny spot with well-drained soil, and space plants 12-18 inches apart. Keep an eye on the weather forecast – even in April, we can get unexpected cold snaps. Buying starts can give you flowers the first year, since they are already established.

Direct sowing Yarrow is a simple and effective method, especially if you're not aiming for first-year blooms. Sow seeds from mid April through late July, once the soil has warmed up a bit. Prepare the soil by loosening it and removing any rocks or debris.

Yarrow prefers well-drained soil, so avoid areas that tend to stay wet. Scatter the seeds lightly over the surface and gently press them into the soil. Keep the soil consistently moist until the seedlings emerge. Thin seedlings to 12-18 inches apart once they have a few sets of true leaves.

Yarrow is remarkably drought-tolerant once established, making it ideal for our dry Mountain West climate. In fact, overwatering is a bigger concern than underwatering. During the first few weeks after planting, water regularly to help the roots establish.

Once established, reduce watering to only when the top 2 inches of soil feel dry to the touch – use the finger test. In general, aim for about ½ inch of water per week, especially during the hottest part of the summer. Water at the base of the plant to avoid wetting the foliage, which can encourage fungal diseases in our low-humidity environment.

Yellowing leaves or drooping, but firm, stems can indicate overwatering, while wilting leaves suggest underwatering. A light layer of gravel mulch around the base of the plants can help retain moisture and improve drainage, which is helpful in our alkaline soils.

Pruning Yarrow is easy and helps encourage continuous blooming. Deadhead spent flowers regularly throughout the growing season to encourage a second flush of blooms. Simply snip off the flower heads just below the lowest petals.

In the fall, after the first frost around early October, cut the plants back to their basal foliage. This will help tidy up the garden and prepare the plants for winter dormancy. Every 2-3 years, you may need to divide the plants to prevent them from becoming too aggressive.

You can expect your first Yarrow harvest in late June through mid October, about 65 days after planting. Harvest the flowers for drying when they are fully open and the colors are vibrant. This is when they contain the most essential oils.

Cut the stems just above a set of leaves, using sharp scissors or pruners. To encourage continued production, harvest regularly throughout the season. Even as our first frost approaches in early October, you can still harvest any remaining blooms for drying. They make a beautiful addition to dried flower arrangements.

Here are some common problems you might encounter with Yarrow in the Mountain West:

Spreads Aggressively

• What it looks like: Yarrow quickly expands beyond its intended area, crowding out other plants.
• What causes it: Rhizomatous roots spread rapidly, especially in well-drained soil. Our intense sun encourages vigorous growth.
• How to fix/prevent it: Plant in containers or use edging to contain the roots. Divide plants every few years. Remove unwanted seedlings promptly.

Powdery Mildew

• What it looks like: White or grayish powdery coating on leaves and stems. Leaves may curl, yellow, and die.
• What causes it: Fungi that thrive in warm days and cool nights, conditions we often experience in the Mountains. Dry leaf surfaces favor it.
• How to fix/prevent it: Improve air circulation by spacing plants adequately and pruning. Remove affected leaves. Neem oil or potassium bicarbonate sprays can help. A milk spray (1 part milk to 9 parts water) is surprisingly effective.

Aphids

• What it looks like: Clusters of tiny green, black, or white insects on stems and leaf undersides. Sticky honeydew residue. Curled or distorted new growth.
• What causes it: Rapid-reproducing sap-sucking insects. Populations can explode in warm weather.
• How to fix/prevent it: A strong water spray can knock them off. Encourage ladybugs and lacewings (natural predators). Use insecticidal soap for heavy infestations.

Mountain West Specific Challenges: Our moderate-at-altitude summer heat and low humidity generally benefit Yarrow, reducing the risk of many fungal diseases. However, the dry rainfall means you'll need to monitor watering carefully, especially during establishment. The intense UV radiation can sometimes bleach the flower colors, so providing some afternoon shade might be beneficial in the hottest locations.

Yarrow is a great companion plant for many garden vegetables and herbs in the Mountain West. It attracts beneficial insects like ladybugs and hoverflies, which help control pests on plants like tomatoes and peppers. Plus, it is drought-tolerant, so it won't compete for water.

Yarrow is also a good companion for herbs like rosemary and thyme, as they share similar growing requirements. It can even benefit strawberries by attracting pollinators. There are no specific plants to avoid planting near Yarrow, but be mindful of its tendency to spread and ensure it doesn't overcrowd its neighbors.