Yarrow in Zone 9A — Northern California

Yarrow is a fantastic addition to any Northern California garden. It's easy to grow, drought-tolerant (perfect for our dry summers), and its feathery foliage and cheerful flowers add beauty and attract beneficial insects. Plus, you can use the flowers for drying, adding a touch of your garden to your home year-round.

While we NorCal gardeners face challenges like drought and potential fire season, Yarrow’s resilience and adaptability make it a perfect fit. With our long, 288-day growing season, you'll have plenty of time to enjoy its beauty and benefits. Careful timing and smart watering will set you up for success.

Starting Yarrow indoors in Northern California is an option, but it's not always necessary given our mild climate and long growing season. If you want a head start, especially to get blooms in the first year, begin in early to late December, about eight weeks before you plan to transplant.

Use seed trays with a good seed-starting mix, and keep them in a warm spot with plenty of light – a sunny windowsill or under grow lights works well. Bottom watering helps prevent damping-off; just set the tray in a shallow dish of water until the soil surface is moist. With our early spring-like conditions, indoor-started plants get a good headstart.

Transplant your Yarrow seedlings outdoors from early February through early March. Before planting, harden them off by gradually exposing them to outdoor conditions for about a week. This helps them adjust to the sunlight and temperature changes.

Space the plants 12-18 inches apart in a sunny spot with well-drained soil. While February and March can bring some rain, be mindful of late frosts, which are still possible. Consider buying starts for first-year flowers.

Direct sowing Yarrow is a simple and effective method in our Northern California climate. You can direct sow from mid-February through mid-September, taking advantage of our long growing season. The soil temperature needs to be consistently above 60°F for optimal germination.

Prepare the soil by loosening it and removing any weeds. Scatter the seeds lightly over the surface, and gently press them into the soil. Keep the soil consistently moist until the seedlings emerge, then thin them to 12-18 inches apart. Direct sowing is easier, but you'll wait longer for flowers.

Yarrow is incredibly drought-tolerant, making it a fantastic choice for water-wise gardening in Northern California. Once established, it needs very little supplemental water. In fact, overwatering is more of a concern than underwatering, as it can lead to floppy growth.

During our dry summers, water deeply but infrequently, allowing the soil to dry out between waterings. A good rule of thumb is to check the soil moisture about 2 inches deep – if it's dry, it's time to water. Generally, about 1 inch of water per week is sufficient during the hottest months. Water at the base of the plant rather than overhead to minimize fungal diseases, especially since our humidity is low.

In the spring and fall, when rainfall is more frequent, you may not need to water at all. Signs of underwatering include wilting leaves, while overwatering can cause yellowing leaves and root rot. Mulching around the base of the plants helps retain moisture and suppress weeds, but avoid piling mulch directly against the stems.

Pruning Yarrow is simple and helps encourage continued blooming. Deadhead spent flowers throughout the growing season to encourage a second flush of blooms. This prevents the plant from putting energy into seed production and directs it toward flower production.

In the fall, after the first frost around early December in NorCal, cut the plant back to its basal foliage. This helps tidy up the garden and prepares the plant for winter dormancy. Every 2-3 years, divide the clumps to prevent them from becoming too aggressive and to rejuvenate the plant.

You can expect your first Yarrow harvest in late April through mid-December, about 65 days after planting. Harvest the flowers for drying when they are fully open and the petals are firm. These are excellent for dried flower arrangements.

To harvest, simply cut the stems at the base of the flower head. You can harvest throughout the growing season, as the plant will continue to produce new flowers. Regular harvesting encourages more blooms. As the first frost approaches in early December, you can harvest any remaining flowers for drying.

Yarrow is generally a low-maintenance plant, but there are a few common problems to watch out for in Northern California.

Spreads Aggressively

• What it looks like: Yarrow can spread quickly via rhizomes, popping up in unwanted areas of your garden.
• What causes it: Our long growing season allows Yarrow plenty of time to spread.
• How to fix/prevent it: Plant Yarrow in a contained area, such as a raised bed, or be prepared to regularly dig up and remove unwanted shoots. Dividing the plant every few years also helps control its spread.

Powdery Mildew

• What it looks like: White or grayish powdery coating on leaves, stems. Leaves curl, yellow, and die.
• What causes it: This fungus thrives in warm days, cool nights, and moderate humidity. Dry leaf surfaces actually favor it. The temperature swings we often see in NorCal can contribute.
• 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: These sap-sucking insects reproduce rapidly, especially in warm weather. Ant colonies may farm them for honeydew.
• How to fix/prevent it: A strong water spray can knock them off. Encourage ladybugs and lacewings (natural predators). Insecticidal soap can be used for heavy infestations.

Northern California Specific Challenges: Our moderate-to-hot summers with low humidity and dry-summer rainfall can actually benefit Yarrow, as it thrives in these conditions. However, be mindful of powdery mildew, which can be exacerbated by our temperature swings. Proper spacing and air circulation are key.

Yarrow makes a great companion plant 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 attracts pollinators that improve fruit set. Herbs like rosemary and thyme also benefit from Yarrow's pest-control abilities. Strawberries also enjoy Yarrow's company, as it can deter pests that target the berries. Avoid planting Yarrow near plants that prefer very rich soil and frequent watering, as Yarrow thrives in drier, less fertile conditions.