Yarrow in Zone 5B — Mountain West

Yarrow is a fantastic addition to any Mountain West garden. Its delicate, feathery foliage and clusters of bright flowers not only add beauty but also attract beneficial insects, improving the health of your entire garden. Plus, the cool nights we get even in summer seem to intensify the colors and fragrance of yarrow, making it a real treat.

Our high altitude, intense sun, and short growing season can be challenging, but yarrow is surprisingly well-suited to our climate. With a little planning, you can easily enjoy its beauty and benefits throughout our 149-day growing season.

Starting yarrow indoors in our region is an option, but honestly, it's often not necessary. If you're eager to get a head start or want to ensure you get blooms in the first year from seed, then go for it. Otherwise, direct sowing works just fine.

If you choose to start indoors, sow seeds mid-February through early 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 – a sunny windowsill or grow lights work well. Bottom watering helps prevent damping off, which can be a problem with seedlings. Remember, spring here in the mountains often arrives late, so be patient!

Transplant your yarrow seedlings outdoors late April through mid 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 helps them adjust to the intense sun and temperature swings we often experience in the spring.

Space your plants 12-18 inches apart to allow for good air circulation. While you can grow yarrow from seed, buying starts from a local nursery is a great way to ensure you get flowers in the first year.

Direct sowing yarrow is a simple and effective method, especially for our short season. Sow seeds early May through mid July, once the soil has warmed up a bit. Yarrow isn't too picky, but a well-drained seedbed will give it the best start.

Simply scatter the seeds on the surface of the soil and gently press them in. Keep the soil consistently moist until the seedlings emerge. Thin the seedlings to 12-18 inches apart as they grow.

Yarrow is a champ when it comes to drought tolerance, which is a huge plus in our dry Mountain West climate. Once established, it needs very little supplemental watering. In fact, overwatering can actually cause problems, leading to floppy growth and potentially root rot.

During the first few weeks after planting, water regularly to help the roots establish. After that, let the soil dry out between waterings. A good rule of thumb is to use the "finger test" – stick your finger about two inches deep into the soil. If it feels dry, it's time to water.

Established yarrow typically needs only 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 cool nights. Signs of underwatering include wilting leaves and stunted growth. Overwatering can cause yellowing leaves and a generally unhealthy appearance. A light layer of mulch around the base of the plant can help retain moisture and suppress weeds, but don't overdo it – yarrow prefers well-drained soil.

Pruning yarrow is pretty straightforward. The main thing is to deadhead spent flowers regularly to encourage continued blooming throughout the season. Simply snip off the flower heads just below the first set of leaves.

Do this as soon as the flowers start to fade. In the fall, after the first frost around early October, cut the plant back to its basal foliage. Yarrow can be a bit aggressive, so divide the plants every 2-3 years to keep them from taking over your garden.

You can expect your first yarrow harvest early July through early October, about 65 days after planting. Harvest the flowers for drying when they are fully open and their color is at its peak. These make excellent dried flowers.

To harvest, simply cut the stems just above a set of leaves. Hang the stems upside down in a cool, dark, and dry place to dry. Regular harvesting encourages continued production throughout the season. As the first frost approaches in early October, harvest any remaining flowers to dry for winter use.

Here are a few common problems you might encounter with yarrow in our region:

Spreads Aggressively

• What it looks like: Yarrow can quickly spread via rhizomes (underground stems), popping up in unwanted areas of your garden.
• What causes it: Our intense sun and well-drained soils encourage vigorous growth, which can lead to yarrow spreading beyond its intended boundaries.
• How to fix/prevent it: Plant yarrow in a contained area or use edging to prevent it from spreading. Divide the plants every 2-3 years to control their size.

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.

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: Rapid-reproducing sap-sucking insects. Populations explode in warm weather. Ant colonies may farm them for honeydew.
• How to fix/prevent it: Strong water spray knocks them off. Encourage ladybugs and lacewings (natural predators). Insecticidal soap for heavy infestations. Neem oil works systemically.

Mountain West Specific Challenges: Our moderate-at-altitude heat, low humidity, and dry rainfall can actually be beneficial for yarrow. The dry conditions help prevent many fungal diseases, but you'll need to be vigilant about watering during establishment and watch out for powdery mildew, which can occur even in our dry climate.

Yarrow is a great companion plant for many vegetables and herbs in our Mountain West gardens. It attracts beneficial insects like ladybugs and lacewings, which prey on pests that can damage your crops.

Good companions include:

• Tomatoes and Peppers: Yarrow attracts beneficial insects that help control pests like aphids and tomato hornworms.
• Herbs: Yarrow's strong scent can deter pests from attacking other herbs like rosemary and thyme.
• Strawberries: Yarrow is believed to improve the flavor of strawberries.

I don't know of any bad companions for Yarrow. It plays well with other plants.