Yarrow in Zone 6A — Pacific Northwest

Yarrow is a fantastic addition to any Pacific Northwest garden. Its cheerful flowers bring color and attract beneficial insects, and its drought tolerance makes it a low-maintenance choice for our mild summers. Plus, the cool nights we get here seem to intensify the fragrance.

While our wet springs and the risk of late blight can be a concern, Yarrow is generally a tough plant that thrives with the right timing. With a growing season of around 148 days, you have plenty of time to enjoy its beauty.

Starting Yarrow indoors gives you a head start, especially if you're aiming for blooms in the first year. Begin in late February through mid March, about eight weeks before you plan to transplant. Use seed trays with a good seed-starting mix, provide warmth (a heat mat is helpful), and ensure plenty of light, especially given our often overcast spring days.

Bottom watering is a great way to keep the soil consistently moist without overwatering. Place the tray in a shallow dish of water and let the soil absorb the moisture from the bottom. This encourages strong root growth and prevents damping off.

Transplant your Yarrow seedlings outdoors in late April through late May, once the risk of hard frost has passed. Before transplanting, harden off your seedlings by gradually exposing them to outdoor conditions for about a week. Start with a few hours of shade and increase the time and sun exposure each day.

Space the plants 12-18 inches apart to allow for their clumping growth habit. Keep an eye on the weather forecast – a late cold snap is always possible in the PNW, so be ready to cover your young plants if needed. Remember that Yarrow is a perennial, so buying starts can give you flowers the first year.

Direct sowing Yarrow is a straightforward option, especially if you are not concerned about getting flowers the first year. Sow seeds directly into the garden in early May through mid July, once the soil has warmed up a bit. Yarrow prefers soil temperatures around 60-70°F for optimal germination.

Prepare the soil by loosening it and removing any weeds. Gently press the seeds into the soil surface, but don't bury them too deeply. Keep the soil consistently moist until the seedlings emerge, then thin them to 12-18 inches apart.

Yarrow is very drought-tolerant once established, which is a huge benefit during our dry summer months in the Pacific Northwest. During the wet springs, you likely won't need to water at all unless you're experiencing an unusually dry spell. Let the rain do its job!

Once the dry summer months arrive, check the soil moisture regularly. Use the "finger test" – stick your finger about two inches deep into the soil. If it feels dry, it's time to water. Aim for about an inch of water per week, either from rainfall or irrigation.

Water at the base of the plant rather than overhead, especially as the foliage fills in. This helps prevent powdery mildew, a common issue in our region. Signs of underwatering include wilting leaves and stunted growth, while overwatering can lead to floppy growth and root rot.

Mulching around your Yarrow plants with compost or shredded bark helps retain moisture and suppress weeds, reducing the need for frequent watering.

Deadheading is key to keeping your Yarrow blooming throughout the season. As soon as the flowers start to fade, snip them off just below the flower head. This encourages the plant to produce more blooms.

In the fall, after the first frost around early October, cut the plant back to its basal foliage (the leaves at the base of the plant). This helps keep the plant tidy and prevents it from becoming too woody. Yarrow can be aggressive, so divide the clumps every 2-3 years to keep it in check.

You can expect your first Yarrow harvest from mid July through mid October, about 65 days after planting. Harvest the flowers for drying when they are fully open and at their peak color. This is when they have the strongest fragrance and the most potent medicinal properties.

To harvest, simply cut the stems just below the flower heads. Bundle the stems together and hang them upside down in a cool, dark, and dry place to dry. Yarrow will continue to produce flowers if you harvest regularly, providing you with a continuous supply throughout the season.

As the first frost approaches in early October, harvest any remaining flowers that are still in good condition. Even if they are not fully open, they will still dry nicely and retain their color.

Yarrow is generally a low-maintenance plant, but here are a few common problems to watch out for in the Pacific Northwest:

Aggressive Spreading:

• What it looks like: Yarrow can spread quickly via rhizomes, forming dense clumps and potentially overtaking other plants in your garden.
• What causes it: Its vigorous growth habit, combined with our mild climate, allows it to thrive and spread.
• How to fix/prevent it: Plant in contained beds or use barriers to restrict root spread. Regularly divide clumps every 2-3 years to control its size.

Powdery Mildew:

• What it looks like: White or grayish powdery coating on leaves, stems, and sometimes flowers. Leaves may 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 by spacing plants properly 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 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.

Pacific Northwest Specific Challenges: Our mild summers and relatively dry-summer humidity can actually be beneficial for Yarrow, reducing the risk of many fungal diseases. However, be vigilant about powdery mildew, especially if air circulation is poor. And while late blight is a serious concern for tomatoes and potatoes, it doesn't typically affect Yarrow.

Yarrow makes a great companion plant in the Pacific Northwest garden.

Good companions:

• Tomatoes and Peppers: Yarrow attracts beneficial insects like ladybugs and lacewings, which prey on aphids and other pests that can plague tomatoes and peppers.
• Herbs: Yarrow's strong scent can deter pests from other herbs, and it attracts pollinators that help with herb seed production.
• Strawberries: Yarrow can help improve the flavor and vigor of strawberries by attracting beneficial insects and improving soil health.

Avoid planting Yarrow near plants that prefer very rich, moist soil, as Yarrow thrives in drier conditions and can compete for resources.