Ask the Gardener: How often should you divide bearded iris?

Ask the Expert Gardening
Bearded iris are pretty carefree plantings.
Bearded iris are pretty carefree plantings. Globe File 2001

What to do this week: Sow fall vegetables such as kale, Swiss chard, radish, mustard greens, Chinese cabbage, parsnip, turnip, peas, radicchio, escarole, collards, beets, spinach, and lettuce. A layer of shade-netting garden fabric, preferably fastened over hoops, will improve germination and young plant growth in the summer heat. Mow natural meadows once now and again in late fall to discourage a takeover by invasive weeds, vines, and woodies. Lightly fertilize annuals and vegetables once a month just before you water or when rain is due. Stop fertilizing perennials and roses so they can start preparing for winter dormancy. Most trees, shrubs, and perennials do not require fertilizing, and lawns need to be fertilized only once a year, in the fall.

Q. Should iris rhizomes (but not the roots) be exposed above the ground?

C.W.O., Malden

A. Yes. Bearded iris roots usually need dividing every four years to keep flowering well. Other than that, they are pretty carefree. Fork up a clump sometime in September and cut it into 4-inch-long segments so each piece has at least one fan of leaves. The beige rhizomes look like ginger root from the produce market. Discard any rhizomes with soft rot or the little worms called “iris borers.’’ Replant the pieces about 18 inches apart horizontally so the side with the small, thin roots is facing downward in the soil and the bald side is face up. You can either shallowly cover the rhizomes with a half inch of soil or leave the topside exposed to the sun, like the back of a whale breaching the sea’s surface. Bearded irises may be good perennials for climate change gardens. They thrive in summer heat and drought. Leave at least 8 inches of leaves and flower stems to promote next year’s blooms.


Q. Which flowers am I supposed to deadhead?

L.P., Milton

A. Cutting or pinching off spent flowers is time-consuming but encourages vigor, neatness, and rebloom for many flowers, particularly annuals. It’s faster to shear back entire plants with many small flowers, such as petunias and sweet alyssum, when they start to flag. Biennials are probably going to die after blooming whether you deadhead them or not, and they often self-seed. A biennial is neither a long-lived perennial nor a short-lived annual; instead it has a surprising two-year life cycle, in which it sprouts from fresh seed one year and then flowers, goes to seed, and dies the second year. They include many old-fashioned flowers, including various species of foxglove, Canterbury bells (campanula), forget-me-not, sweet William, columbine, poppy, hollyhock, money plant (Lunaria annua), stock (matthiola), dame’s rocket, wallflower, and feverfew. The dreaded garlic weed is also on a two-year cycle. Most biennials produce a lot of seeds that germinate in the garden without help, producing the charming random appearance of a cottage garden. One advantage of biennials is that many bloom in May and early June after bulbs but before most annuals and perennials hit their stride, filling a hole in the sequence of bloom. Also, they bring an element of surprise to the garden, as you don’t know where they are going to pop up, and some even have different colors than their parents. I won the genetic lottery when white foxgloves bloomed in my garden this year. I deadheaded the common pink parents, so only their white offspring could reproduce.

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