Yarrow in Zone 4B — Mountain West

Yarrow is a workhorse in the Mountain West garden. It's a beautiful and useful plant, bringing color and attracting beneficial insects to your garden. Plus, the cool nights even in summer here seem to intensify its fragrance, making it a joy to have around.

Our short season and alkaline soil can be a challenge for some plants, but Yarrow is well-suited to our climate. With a 128-day growing season, you'll have plenty of time to enjoy its blooms. Timing is key.

Starting Yarrow indoors isn't strictly necessary, but it can give you a head start in our short growing season. If you're aiming for blooms in the first year, it can be worth the effort.

Start your seeds indoors from late February through mid March, about eight weeks before you plan to transplant. Use seed trays with a good seed-starting mix, and keep them warm and well-lit. Bottom watering is a great way to keep the soil moist without disturbing the tiny seeds. Remember that spring here in the Mountains often starts late, so don't rush things.

Transplanting Yarrow outdoors is best done from early to late May, after the danger of hard frost has passed. Before you move your seedlings outside, harden them off for about a week. This means gradually exposing them to outdoor conditions so they don't get shocked by the intense sun and cool nights.

Space your plants 12-18 inches apart, as they will spread. While Yarrow is a perennial, buying starts from the garden center can let you enjoy the flowers in the first year. Keep an eye on the weather; unexpected late frosts can still happen in May.

Direct sowing Yarrow is a viable option, especially if you're not in a rush for first-year blooms. The key is waiting for the soil to warm up.

Sow your seeds from mid May through early July. Prepare the soil by loosening it and removing any large clumps. Yarrow prefers well-drained soil, so avoid areas that stay soggy. Lightly press the seeds into the soil, but don't bury them too deep. Space them about 12-18 inches apart.

Yarrow thrives on neglect, which is perfect for our dry Mountain climate. Once established, it's very drought-tolerant. Overwatering is a bigger problem than underwatering, and can lead to floppy growth.

During the first few weeks after planting, water regularly to help the roots establish. After that, ease up. A good rule of thumb is to check the soil with your finger about two inches deep. If it's dry, it's time to water. In general, you might only need to water established plants about once a week, giving them around half an inch of water each time. Water at the base of the plant to avoid wetting the foliage, which can encourage fungal diseases in our low-humidity environment.

Signs of underwatering include wilting or yellowing leaves, while overwatering can cause yellowing and droopy leaves. Mulching around your plants with gravel or wood chips can help retain moisture in the soil and suppress weeds.

Pruning Yarrow is simple and helps keep it looking its best. The main task is deadheading spent flowers.

Deadhead throughout the growing season, from mid July through late September, to encourage more blooms. Simply snip off the faded flower heads just below the flower. In the fall, after the first frost around mid- September, cut the plant back to its basal foliage (the leaves at the base of the plant). Yarrow can be aggressive, so divide the plants every 2-3 years to keep them in check.

You can expect your first harvest of Yarrow flowers from mid July through late September, about 65 days after planting. The best time to harvest is when the flowers are fully open and brightly colored.

To harvest, simply cut the stems just below the flower heads. You can harvest throughout the season as the flowers bloom. Cutting the flowers encourages the plant to produce more. Near the end of the season, as the first frost approaches in mid- September, harvest any remaining flowers to dry. Yarrow makes an excellent dried flower, and it attracts beneficial insects to your garden.

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

Aggressive Spreading

• What it looks like: Yarrow can quickly spread beyond its intended area, forming dense clumps and crowding out other plants.
• What causes it: Yarrow spreads through rhizomes (underground stems), which can easily take over if left unchecked. Our intense sun and well-drained soils can actually help it thrive.
• How to fix/prevent it: Plant Yarrow in a contained area, such as a raised bed or surrounded by a barrier. Divide the plants every 2-3 years to control their spread. Regularly remove unwanted shoots that pop up.

Powdery Mildew

• What it looks like: White or grayish powdery coating on leaves and stems. Leaves may curl, yellow, and die.
• What causes it: Powdery mildew is caused by 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 by spacing plants adequately and pruning dense growth. Remove affected leaves promptly. Neem oil or potassium bicarbonate sprays can help. A milk spray (1 part milk to 9 parts water) is also surprisingly effective.

Aphids

• What it looks like: Clusters of tiny soft-bodied insects (green, black, or white) on stems and leaf undersides. Sticky honeydew residue. Curled or distorted new growth.
• What causes it: Aphids are sap-sucking insects that reproduce rapidly, especially in warm weather. Ant colonies may farm them for honeydew.
• How to fix/prevent it: A strong spray of water can knock aphids off plants. Encourage ladybugs and lacewings (natural predators). Insecticidal soap can be used for heavy infestations. Neem oil works systemically.

Mountain West Specific Challenges: The moderate-at-altitude heat and low humidity of the Mountain West generally benefit Yarrow, reducing the risk of many fungal diseases. However, our intense UV radiation can stress plants, making them more susceptible to pests like aphids. Be sure to provide adequate water during dry spells, but avoid overwatering.

Yarrow makes a great companion plant for many vegetables and herbs in the Mountain West garden.

Good companions include tomatoes and peppers because Yarrow attracts beneficial insects like ladybugs and lacewings, which prey on common pests that bother those crops. It also works well with herbs like rosemary and thyme, as they share similar growing requirements (full sun and well-drained soil). Strawberries also benefit from Yarrow's pest-repelling properties and its ability to improve soil health.

Avoid planting Yarrow near cabbage or other brassicas, as it can attract cabbage moths, which will then feed on your vulnerable brassica leaves.