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Why won’t my iris bloom?

An Iris flower at the Meiji Shrine inner garden in Tokyo, Japan.Kiyoshi Ota/EPA

What to do this week As the pace of chores slows down, watering and general maintenance become the main activities. Cut off withering and dead flowers and leaves to keep plants from going to seed and to give the garden a fresh appearance. Stake everything that needs it, including tomato, hollyhock, dahlia, delphinium, and hibiscus. Allow biennials such as lupine, foxglove, columbine, forget-me-not, sweet William, and hollyhock to go to seed to produce new plants for next year. But don’t let phlox go to seed, or the pretty parents will inevitably be crowded out by more vigorous progeny with inferior colors. Phlox is one of those perennials that will form more flowers if you cut back the spent ones. Others that provide this bonus include salvia, heliopsis, delphinium, and Rozanne geranium. Sow seeds of beet, cabbage, kale, carrot, bean, turnip, and radish directly into the garden for late-season crops.

Q. I planted irises nearly 20 years ago in my garden in Lowell. They have been divided many times. For the past two years, there have been minimal blooms. Can you explain why?

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JUDITH, Lowell

A. There are many species of irises, but I assume you are asking about so-called German or bearded irises, named after the Greek rainbow goddess. They bloom in every combination of colors. Henry Mitchell, the late esteemed garden columnist at The Washington Post, wrote that he took vacation time when his irises were in bloom, so he could stay home and admire them at his leisure. One big problem with irises, as you know, is that to flower well, the roots (called rhizomes) need to be lifted out of the ground about every four years, cut or broken into smaller pieces, each attached to at least one fan of leaves, and then replanted 18 inches apart in a location that gets at least six hours of direct sun a day.

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Each rhizome is replanted horizontally so the side with the dangling roots is facing down and the bald side is facing up, level with and almost running along the top of the surface of the soil. Irises often don’t bloom if they are planted too deeply or mulched, so dividing and replanting the roots gives you the opportunity to correct this. Now through early fall is a good time to divide iris rhizomes. Since they are so shallow-rooted, you can dig them up with a trowel and break the roots into pieces with your fingers. Then check them for disease or pests such as iris borers, discard any rotting rhizomes, and move them to a sunnier location if their old location has gotten too shady. Replant and water them immediately. Though bearded irises survive drought easily, one inch of rain or irrigation once a week promotes blooms the following year.

Maybe your irises did not bloom because they got too little (or too much) water last year. Too much nitrogen can also be a problem. Use a fertilizer with a low first number, such as 5-10-10, twice a year. Nitrogen promotes leaf growth, while the second number refers to phosphorus, which fosters flowering. The third number stands for potassium, which prepares plants for summer drought and winter cold. For more flowers, try sprinkling a half cup of 10-55-10 fertilizer around (but not on top of) the iris rhizomes now to promote blooming next year, and do it again next April.

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Your problem could also be iris borers, which is the reason I don’t have many bearded irises, though I do grow a lot of other types, such as crested, Louisiana, Japanese, plum tart, and Siberian. All iris species can get borers, but bearded ones seem to be the most susceptible in my experience. Since iris borer moths lay their eggs in leaves in the fall, cut them off after the first frost and clean up the dead leaves around them, removing them from your property.

Q. My rose of Sharon tree did not do well over the winter. The upper branches appear to be dead (dry and brown, no new leaves), but the lower part is sprouting lots of new growth (outward rather than upward). Should I cut off the top branches? Will the lower part eventually grow upward? Any advice would be appreciated.

ANNE C., Cambridge

A. Worry not. Rose of Sharon (Hybiscus syriacus) is not the delicate creature it sometimes appears to be. In fact, it is as rough and tough as a forsythia. Which doesn’t mean that dieback is an impossibility. My rose of Sharon did the same thing as yours: Some of the top branches appear dead, while there are leaves now sprouting on the trunks and lower branches, like you said. But I’m not taking it too seriously. Rose of Sharon doesn’t normally leaf out until June anyway, so people think it is dead when it is just fine. In fact, I think it is impossible to kill. As far as appearance goes, I’m going to leave mine alone for now, and I suggest you do the same and see what happens. I think the lower branches will grow out and up and cover any dead area. A mature shrub will produce many dozens of new stems over the summer, enough to cover any dead zone.

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This is one shrub that doesn’t require pruning for health, but it will grow 10 feet tall by 10 feet wide in 10 years if it isn’t cut back. Because of its vigor and size, it makes a good summer flowering hedge for a vacation home, but not a good foundation planting. I just shape mine by hacking it back by about a third every couple of years between March and May because, like everyone else, I planted it too close to a building before I realized how big it wants to get. In exasperation, I once cut a rose of Sharon back to a three-foot stump. It grew back just fine.

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