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ASK A MASTER GARDENER: Tending to irises and hydrangeas

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Q: I have two limelight hydrangeas. One is in a shaded area next to the house and is full of leaves. The other, in the heart of the garden, has only a few leaves beginning to emerge. Any suggestions as to how to bring it up to speed?

A: This is a great example of micro-climates. The hydrangea next to your house benefitted from its protection, while the other was probably several degrees colder. The plant next to your house felt spring way before the other and so got its head start. Even a low spot in a yard can trap cold air and be cooler than the rest of the garden. There isn’t much we can do to change that without resorting to greenhouses or other heat-catching devices. If the slower plant is getting enough sun, water and fertilizer, the only thing it needs more of its time.

Q: My neighbor gave me some irises. They bloomed beautifully in his yard, but in mine they just send up green shoots and don’t bloom. What have I done wrong?

A: If you just got the iris from him, they may need a year to settle in. Irises usually don’t bloom for a year after being transplanted. If you’ve had them longer than that and they’re still not performing for you, the problem could be where they are planted.

Irises don’t bloom well in shady locations. They want at least six hours of sun a day. They also may fail to bloom if they are planted too deep. The tops of the rhizomes should be at or just below the surface of the soil. A heavy mulch on top of the rhizomes will slow blooming, too. (It’s a good idea to cover iris with leaves or other mulch for the winter, but the mulch should be pulled off in spring.)

Older, established clumps of iris may stop blooming if they get too crowded. They need to be dug up and divided every few years.

I’m not sure what kind of iris you have, but bearded iris want well-drained soil. Siberian iris and blue flag or yellow flag iris can tolerate moist conditions, but bearded iris may fail to bloom if the soil is too wet.

Iris may also give you green shoots and no flowers if they’ve had too much nitrogen fertilizer. And iris may fail to bloom if there is a late freeze.

There are also some pests and diseases that will inhibit blooming, but I think the leaves would look unhappy if that were what’s holding yours back.

If they’re just in the wrong spot or planted too deep, you can lift and replant them, but it may take a year before they look as nice as your neighbor’s.