Yarrow in Zone 6B — Mountain West

Yarrow is a fantastic addition to any Mountain West garden. Its feathery foliage and clusters of vibrant flowers bring beauty and attract beneficial insects, making it perfect for supporting a healthy garden ecosystem. Plus, the cool nights even in summer enhance the essential oils, resulting in a more fragrant and flavorful plant.

Our high altitude gardening comes with unique challenges like intense sun and a short season. But with careful planning and timing, you can successfully grow Yarrow. Our growing season is about 163 days, so let's maximize it.

Starting Yarrow indoors gives you a head start, especially with our short growing season. Begin in early to late February, about 8 weeks before you plan to transplant. This is most useful if you want flowers in the first year.

Use seed trays or small pots filled with a well-draining seed starting mix. Keep the soil consistently moist, using bottom watering to avoid disturbing the tiny seeds. Provide warmth and plenty of light – a sunny windowsill or grow lights work well. Remember that our late springs mean you need to be patient and avoid setting out plants too early after a warm spell.

Transplant your Yarrow seedlings outdoors in mid April through early May, once the danger of hard frost has passed. Before transplanting, harden off your seedlings by gradually exposing them to outdoor conditions for about a week. This helps them adjust to the intense sun and cooler temperatures.

Choose a sunny spot with well-drained soil, spacing plants 12-18 inches apart. Even in April and early May, keep an eye on the weather forecast. Be ready to cover your young plants if a late-season snow or hard freeze threatens. Since Yarrow is a perennial, consider buying starts if you want flowers the first year.

Direct sowing Yarrow is a viable option, especially if you're not concerned about getting flowers in the first year. Sow seeds directly into the garden from late April through mid July. Soil temperatures need to be consistently above 60°F for good germination.

Prepare the soil by loosening it and removing any weeds. Scatter the seeds thinly over the surface, and gently press them into the soil. Keep the soil consistently moist until seedlings emerge. Thin seedlings to 12-18 inches apart once they have a few sets of true leaves. Direct sowing is less work, but you might not get blooms the first year in our short season.

Yarrow is incredibly drought-tolerant once established, making it a great choice for our dry Mountain West climate. However, it does need consistent watering while getting established. During the first few weeks after planting, water deeply whenever the top inch or two of soil feels dry.

After that initial period, cut back on watering significantly. A good rule of thumb is to water only when the top 2 inches of soil are dry to the touch – use the "finger test." In our region, this might mean watering deeply every 1-2 weeks during the hottest part of the summer, but less frequently during cooler periods. Aim for about 1 inch of water per week, either from rainfall or irrigation.

Water at the base of the plant to avoid wetting the foliage, which can increase the risk of powdery mildew in our cool nights. Signs of underwatering include wilting leaves and stunted growth. Overwatering, on the other hand, can lead to floppy growth and root rot. A light mulch of gravel or shredded bark can help retain moisture in the soil while also promoting good drainage.

Regular pruning helps keep your Yarrow plants healthy and encourages more blooms. Deadhead spent flowers throughout the growing season, snipping off the flower heads just below the lowest set of petals. This prevents the plant from putting energy into seed production and encourages a second flush of flowers.

In the fall, after the first frost around early October, cut the plants back to their basal foliage. This helps tidy up the garden and prepares the plants for winter dormancy. Yarrow can be aggressive, so divide the clumps every 2-3 years in the spring to prevent them from overcrowding your garden.

You can expect your first harvest of Yarrow flowers from late June through mid October, depending on when you planted and the specific variety. Harvest the flowers for drying when they are fully open and the petals are still vibrant. The flowers make excellent dried arrangements and retain their color well.

To harvest, simply cut the flower stems with a sharp knife or scissors. Cut long stems to encourage more branching. Continued deadheading and harvesting will encourage continuous blooming throughout the season.

As the first frost approaches in early October, harvest any remaining flowers before they are damaged by the cold. You can dry them for later use or simply enjoy them fresh in bouquets. The plant attracts many beneficial insects, so consider leaving some flowers in the garden to provide habitat for them.

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

Aggressive Spreading

• What it looks like: Yarrow spreads quickly via rhizomes, forming dense clumps that can crowd out other plants.
• What causes it: Yarrow thrives in our sunny conditions and can become overly enthusiastic, especially in fertile soil.
• How to fix/prevent it: Plant Yarrow in a contained area or use barriers to prevent it from spreading. Divide the clumps every 2-3 years to keep them in check.

Powdery Mildew

• What it looks like: White or grayish powdery coating on leaves, stems, and sometimes fruit. Leaves curl, yellow, and die.
• What causes it: Various fungi that thrive in warm days, cool nights, and moderate humidity. Dry leaf surfaces actually favor it.
• 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.
• 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 combined with low humidity generally reduces disease pressure on Yarrow, but powdery mildew can still occur with cool nights. The dry rainfall means you'll need to provide supplemental water during dry spells, especially while plants are establishing.

Yarrow makes a great companion plant in the Mountain West garden. It attracts beneficial insects like ladybugs and lacewings, which help control pests on other plants.

• Tomatoes & Peppers: Yarrow attracts beneficial insects that prey on tomato hornworms and aphids, common pests in our region.
• Herbs: Planting Yarrow near herbs like rosemary and thyme can help improve their flavor and vigor.
• Strawberries: Yarrow can help deter pests that attack strawberries, such as spider mites.

Avoid planting Yarrow near plants that prefer rich, moist soil. Its drought tolerance and preference for well-drained soil make it an unsuitable companion for plants with high water needs.