Yarrow

Yarrow is one of the few garden plants that actually suffers from good treatment. A yarrow planted in rich, well-amended soil with regular watering tends to grow tall, weak-stemmed, and floppy — the stems sprawl in the first summer rainstorm and the flower production drops. The same plant in lean, dry soil stands upright, blooms heavily, and needs no staking. If you're accustomed to feeding and watering everything, yarrow is the plant that will teach you the value of calculated neglect.

The flat-topped flower clusters that bloom from early summer through fall are landing pads for an enormous range of beneficial insects. Parasitic wasps, predatory beetles, hoverflies, and lacewings all feed on the nectar, and many of them spend the rest of their time hunting aphids, caterpillars, and other garden pests. A patch of yarrow in bloom is essentially a recruiting station for your garden's pest control crew.

The challenge with yarrow is not getting it to grow — it's keeping it where you want it. The plant spreads by rhizomes and self-seeds freely, which can be an asset in a naturalistic meadow planting or a problem in a formal perennial border. The center of an established clump tends to die out after a few years, leaving a ring of healthy growth around a bare patch. Dividing every two to three years keeps the clumps vigorous and gives you a chance to relocate or discard the extras before they colonize the entire bed.

Deadheading extends the bloom period and reduces self-seeding, but it's not strictly necessary — yarrow is one of those plants that looks fine even when it goes to seed, and the dried flower heads hold their structure through winter. If you want continuous bloom, cut the stems back by half after the first flush of flowers; new stems will come up and bloom again in late summer.

Yarrow can be started from seed, but germination tends to be uneven and the seedlings are slow to reach blooming size. Buying a transplant or dividing an established clump is faster. Set plants out in spring after the last frost, water them in, and then largely ignore them. Once established, yarrow rarely needs supplemental water except in the most severe droughts, and even then it will often survive on its own.