Yarrow in Zone 7A — Northern California

Yarrow is a fantastic choice for NorCal gardens. Its vibrant flowers add beauty, attract beneficial insects, and can be dried for arrangements. Plus, it's incredibly easy to grow here, flourishing in our long, sunny days and tolerating our often poor soils.

We Northern California gardeners know about drought and the challenges of water restrictions. Yarrow is a winner because it thrives with minimal watering once established. With our long 235-day growing season, you'll have plenty of time to enjoy its blooms.

Starting yarrow indoors is an option, but honestly, it's usually not necessary in our climate. Our early springs mean you can direct sow or transplant easily. If you're impatient for blooms, starting seeds indoors from early to late January gives you a head start.

Use seed trays with a good seed-starting mix. Keep them warm (around 70°F) and provide plenty of light – a grow light works wonders. Bottom watering helps prevent damping off; set the tray in a shallow dish of water until the surface of the soil is moist.

Transplanting yarrow is a straightforward way to get a jump on the season. Aim to transplant outdoors from early March through early April, once the soil has warmed up a bit.

Before planting, harden off your seedlings. Give them about a week of gradual exposure to the outdoors, increasing the time each day. Space the plants 12-18 inches apart. Keep an eye on the weather – those early spring days can still bring unexpected cold snaps. If you want flowers the first year, consider buying starts from a local nursery.

Direct sowing yarrow is probably the easiest method for us here in Northern California. The soil warms up quickly, and the seeds germinate readily. Sow directly from mid March through late August for a continuous supply of blooms.

Prepare the soil by loosening it and removing any weeds. Scatter the seeds lightly over the surface, then gently rake them in. Keep the soil moist until the seedlings emerge. Thin the seedlings to 12-18 inches apart once they have a few sets of true leaves.

Yarrow is incredibly drought-tolerant, making it perfect for our dry summers and water-wise gardening practices. Once established, it needs very little supplemental watering. However, getting it established requires some attention, especially in our Mediterranean climate.

During the spring, water regularly to help young plants establish their root systems. Once the weather heats up in summer, scale back. Stick your finger about 2 inches deep into the soil. If it feels dry, it's time to water. Aim for about 1 inch of water per week during the hottest periods, but only if needed.

Water at the base of the plant to avoid wetting the foliage, as this can encourage fungal diseases, even with our low humidity. Mulching around the base of the plants with straw or wood chips helps retain moisture and suppresses weeds – a must for our long, hot summers. Overwatering leads to floppy growth, while underwatering results in stunted growth and fewer flowers.

Pruning yarrow is simple and keeps the plant looking its best. Deadheading spent flowers encourages a second flush of blooms.

Simply snip off the flower heads just below the lowest set of leaves. Do this throughout the growing season, as needed. In the fall, after the first frost around early November, cut the plant back to its basal foliage (the leaves at the base). Since yarrow can be aggressive, divide the clumps every 2-3 years to keep it in check.

You can expect your first yarrow harvest from late May through mid November, about 65 days after planting. Harvest the flowers for drying when they are fully open and the colors are vibrant.

Cut the stems at the base of the plant, taking care not to damage the surrounding foliage. To encourage continued production, harvest regularly. As the first frost approaches in early November, harvest any remaining flowers to dry indoors. Yarrow doesn't ripen off the vine, so only harvest fully open blooms.

Yarrow is generally easy to grow, but here are a few common problems we see in Northern California:

Spreading Aggressively

• What it looks like: Yarrow spreads via rhizomes (underground stems), quickly taking over garden beds.
• What causes it: Our long growing season and mild winters allow yarrow to grow almost year-round, leading to rapid spread.
• How to fix/prevent it: Plant in containers or use edging to contain the roots. Divide clumps regularly (every 2-3 years) to prevent overcrowding. Remove unwanted plants promptly.

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. Plant resistant varieties.

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. Avoid excess nitrogen fertilizer (it attracts them).

Northern California Specific Challenges: Yarrow is fairly well-suited to our moderate-to-hot heat, low humidity, and dry-summer rainfall. The key is to avoid overwatering, especially as our water restrictions become more common. Choose well-draining soil and mulch to retain moisture without waterlogging the roots.

Yarrow is a great companion plant for many vegetables and herbs in our Northern California gardens. It attracts beneficial insects that prey on pests.

Good companions:

• Tomatoes: Yarrow attracts hoverflies and ladybugs, which control aphids that can plague tomatoes.
• Peppers: The same beneficial insects that help tomatoes also benefit peppers.
• Herbs: Yarrow supports the growth of many herbs, like rosemary and thyme, by improving soil conditions and attracting pollinators.
• Strawberries: Yarrow can help deter pests that attack strawberries.

I'm not aware of any plants that are specifically bad companions for yarrow.