Yarrow in Zone 6B — Pacific Northwest

Yarrow is a workhorse in the PNW garden. Its cheerful flowers bring pops of color to our mild summers, plus it's a magnet for beneficial insects. And because our cool nights intensify flavors, you'll find yarrow's herbal notes are especially pronounced.

While we PNW gardeners might fret about late blight and cool spring soil, yarrow is pretty unfazed. Its adaptability and relatively quick maturity (65 days) mean you can easily fit it into our 162-day growing season.

Starting yarrow indoors gives you a head start, especially if you want blooms the first year. Sow seeds indoors from mid February through early March, about 8 weeks before transplanting. Use seed trays with a good seed-starting mix and keep them consistently moist by bottom watering.

Provide warmth and light – a heat mat and grow lights are helpful given our overcast spring days. Remember, though, direct sowing works too, so indoor starting isn't essential.

Transplant your yarrow seedlings outdoors from mid April through mid May, after hardening them off for about a week. This gradual exposure to the elements helps them adjust to the stronger sunlight and cooler temperatures. Space them 12-18 inches apart to allow for their clumping growth habit.

Keep an eye on the weather forecast; a late frost can still happen, even in late April. Buying starts is a great way to ensure you get flowers the first year.

Direct sowing yarrow is easy and effective, especially if you're not in a rush for first-year blooms. Sow seeds directly into well-prepared soil from early May through late July. Yarrow prefers soil temperatures around 60-70°F for optimal germination.

Loosen the soil and lightly cover the seeds, spacing them about 12-18 inches apart. Keep the soil consistently moist until seedlings emerge. Direct sowing is a great, low-effort way to establish a patch of yarrow.

Yarrow is wonderfully drought-tolerant once established, making it perfect for our dry summer months. During the wet springs here in the PNW, you'll barely need to water at all. In fact, overwatering is more of a concern than underwatering.

During our mild summers, check the soil moisture with the finger test – if the top 2 inches are dry, it's time to water. Aim for about an inch of water per week during dry spells. Water at the base of the plant to avoid creating a humid environment around the foliage, which can encourage powdery mildew.

Signs of underwatering include wilting or drooping leaves. Overwatering, on the other hand, can lead to floppy growth and root rot. A light mulch of gravel can help retain moisture without water-logging the soil.

Deadheading is key to keeping your yarrow blooming throughout the season. Snip off spent flower heads as soon as you notice them fading, which encourages the plant to produce more blooms. Do this regularly from early July through mid October.

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 clumps every 2-3 years to keep it in check.

You can expect your first yarrow harvest from early July through mid October, about 65 days after planting. Harvest flowers when they are fully open and their color is vibrant. These are excellent for drying and using in crafts.

Cut the stems cleanly with sharp scissors or pruners, leaving enough foliage on the plant to continue growing. Regular harvesting encourages continued production. Even as the first frost approaches in early October, you can still harvest any remaining flowers for drying.

Yarrow is generally low-maintenance, but here are a few potential problems to watch out for in the PNW:

Aggressive Spreading:

• What it looks like: Yarrow popping up in unwanted areas of your garden.
• What causes it: Its rhizomatous roots spread easily, especially in well-drained soil.
• How to fix/prevent it: Plant in a contained area or divide regularly (every 2-3 years) to keep it in check. Be vigilant about removing unwanted seedlings.

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, cool nights, and moderate humidity – conditions we often experience in the PNW.
• 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 a surprisingly effective remedy.

Aphids:

• What it looks like: Clusters of tiny green, black, or white insects on stems and undersides of leaves. Sticky honeydew residue. Curled or distorted new growth.
• What causes it: Sap-sucking insects that reproduce rapidly, especially during our mild summers.
• How to fix/prevent it: Knock them off with a strong spray of water. Encourage beneficial insects like ladybugs and lacewings. Use insecticidal soap for heavy infestations.

Pacific Northwest Specific Challenges: While late blight isn't a direct threat to yarrow, our cool, wet springs can contribute to conditions favorable for powdery mildew. Our dry-summer climate, however, means you likely won't have to worry about many other fungal diseases.

Yarrow makes a great companion plant in the PNW garden. It's particularly beneficial near tomatoes and peppers, attracting beneficial insects that prey on common pests. Herbs like rosemary and thyme also benefit from yarrow's presence.

Yarrow is also a good companion for strawberries. Its ability to attract beneficial insects can help control strawberry pests. There are no specific plants to avoid planting near yarrow.