Yarrow in Zone 7B — Northern California

Yarrow is a fantastic addition to any Northern California garden. Its cheerful blooms bring color and texture, plus it's a magnet for beneficial insects. Growing yarrow gives you armloads of flowers for drying, and that spicy-sweet scent is just lovely in the garden.

We NorCal gardeners face our share of challenges, from drought to unpredictable weather. But yarrow's a tough cookie, and with our long growing season, you'll have plenty of time to get it established. You can easily harvest from mid-May all the way through late November, enjoying about 245 days of growth.

Starting yarrow indoors isn't strictly necessary here in Northern California, but it can give you a jump start, especially if you're aiming for blooms in the first year. If you want to go this route, start your seeds indoors from late December through mid January, about eight weeks before you plan to transplant.

Use seed trays with a good seed-starting mix, and provide warmth and light. A heat mat underneath the trays can help with germination, and a grow light ensures strong, healthy seedlings. Don't forget to water from the bottom to avoid damping off.

Transplant your yarrow seedlings outdoors from early to late March, once the threat of frost has passed. Before planting, be sure to harden off your seedlings by gradually exposing them to outdoor conditions for about a week. This helps them adjust to the sun and wind.

Plant them 12-18 inches apart in a sunny spot with well-drained soil. Even though our springs are generally mild, keep an eye on the forecast for any late cold snaps. Since Yarrow is a perennial, buying starts can help guarantee flowers the first year.

Direct sowing yarrow is a perfectly viable option in our Northern California climate, especially if you're not in a rush for first-year blooms. Sow seeds from mid March through late August. This gives them plenty of time to germinate and establish before winter.

Prepare the soil by loosening it and removing any weeds. Yarrow prefers well-drained soil, so amend with compost if needed. Sprinkle the seeds lightly on the surface, and gently press them into the soil. Keep the soil moist until germination, which usually takes about 10-14 days. Thin seedlings to 12-18 inches apart.

Yarrow is incredibly drought-tolerant once it's established, making it a great choice for water-wise gardening in Northern California. In our Mediterranean climate, with its dry summers and moderate-to-hot heat, proper watering is key, especially during the first year.

During the spring, water deeply about once a week, providing around 1 inch of water. Use the "finger test" to check the soil moisture – if the top 2 inches are dry, it's time to water. As the summer heat kicks in, you might need to increase watering to twice a week, especially if you're experiencing water restrictions.

Water at the base of the plant to avoid wetting the foliage, which can encourage fungal diseases in our low-humidity environment. Signs of underwatering include wilting leaves and stunted growth, while overwatering can lead to floppy growth and root rot. A layer of mulch around the base of the plant helps retain moisture and suppress weeds.

Pruning yarrow is simple and keeps the plant looking its best. The main thing is to deadhead spent flowers regularly throughout the growing season. This encourages a second flush of blooms and prevents the plant from self-seeding too aggressively.

Simply snip off the faded flower heads just below the next set of leaves. Towards the end of the season, after the first frost around mid- November, cut the plant back to its basal foliage.

You can expect your first yarrow harvest from mid May through late November here in Northern California. Harvest the flowers for drying when they are fully open and the colors are vibrant. These make excellent dried flowers for arrangements.

To harvest, simply cut the stems just above a set of leaves. This encourages the plant to produce more flowers. Yarrow is a continuous harvest plant, meaning you can keep harvesting throughout the growing season as long as you keep deadheading.

As the first frost approaches in mid- November, harvest any remaining flowers before they are damaged by the cold. Don't worry if you miss some – the plant will come back strong next year.

Yarrow is generally a low-maintenance plant, but there are a few common problems to watch out for in Northern California: aggressive spreading, powdery mildew, and aphids.

Aggressive Spreading

• What it looks like: Yarrow can quickly spread via rhizomes, forming large clumps and potentially overtaking other plants in your garden.
• What causes it: Our long growing season and mild winters allow yarrow to thrive and spread rapidly.
• How to fix/prevent it: Plant yarrow in a contained area or in pots. Divide the clumps every 2-3 years to control its spread.

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: Powdery mildew thrives in warm days, cool nights, and moderate humidity. It's more common in late summer and early fall when the weather starts to cool down.
• How to fix/prevent it: Improve air circulation by spacing plants adequately and pruning dense foliage. Remove affected leaves promptly. You can also try neem oil or potassium bicarbonate sprays. 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: Aphids are sap-sucking insects that thrive in warm weather. They can quickly multiply and cause significant damage to plants.
• How to fix/prevent it: Knock aphids off plants with a strong stream of water. Encourage beneficial insects like ladybugs and lacewings, which are natural predators of aphids. For heavy infestations, use insecticidal soap or neem oil.

Northern California Specific Challenges: Our moderate-to-hot heat, low humidity, and dry summers can stress yarrow if it's not properly watered, making it more susceptible to pests and diseases. Be sure to provide adequate water, especially during the first year, and monitor your plants regularly for any signs of problems.

Yarrow makes a great companion plant for many vegetables and herbs in the Northern California garden. It attracts beneficial insects like ladybugs and lacewings, which help control pests on other plants.

Good companions include tomatoes and peppers, as yarrow helps to repel pests that commonly attack these crops. It's also a good companion for herbs like rosemary and thyme, as it helps to improve their growth and flavor. Strawberries also benefit from yarrow, as it attracts pollinators and helps to deter pests. Avoid planting yarrow near brassicas like cabbage and broccoli, as it can attract aphids, which can then spread to these crops.