California/common yarrow (Achillea millefolium) is a spreading perennial with fernlike aromatic leaves and flat clusters of tiny flowers. It occurs naturally across California and is useful as a low-water meadow or groundcover plant; its flowers attract bees, butterflies, and other beneficial insects.

Growing it

• Light: Full sun gives the densest growth and flowering, though yarrow tolerates some part shade.
• Water: Keep new plants reasonably moist while roots establish. Once established it is drought tolerant; inland California plantings may benefit from occasional summer water if you want them to stay green and spread.
• Soil: Yarrow tolerates poor soil surprisingly well. Drainage matters more than fertility: rich, wet conditions tend to produce tall, floppy growth.
• pH: Do not obsess over it. Yarrow is broadly adaptable; a roughly neutral soil is fine.
• Habit: It spreads by rhizomes and can form colonies. That is an advantage when you want a carpet and a nuisance where you want a tidy specimen.

What the plant is telling you

• Too much water / overly rich soil: luxuriant, tall growth that flops.
• Too little water: foliage loses vigor, browns, or dies back; established plants survive drought better than new transplants.
• Happy: dense ferny foliage, outward spread, then flower stalks rising above the mat.

For Gavin's yard

For a soft-ish, low-water California groundcover, yarrow is a strong candidate. Establish it with regular water, then reduce irrigation. More summer water generally means greener, faster-spreading growth; less water gives a tougher, more dormant-looking summer plant.