Our beautiful spring flowers have faded away. Now what should be done?

Here are some thoughts on a few of the more common plants that bloom in spring in the Lehigh Valley.

Don’t trim iris leaves. Leaves carry on photosynthesis for next year’s growth. Cut off brown tips and cut the flowering stalk down to the rhizome (fleshy roots) to discourage rot. After two to five years, when clumps become congested or lose vitality and stop blooming, divide and replant sound rhizomes in fresh soil. The best time to replant irises is soon after they bloom. Transplant them to places where they will have “wet feet, but dry knees.”

Avoid applying high-nitrogen fertilizers to the surface of iris beds or carelessly mulching with organic matter, which may encourage rhizome rot. Keep rhizomes exposed. Unlike bulbs, which thrive deep underground, iris rhizomes need a bit of sun and air to dry them out. If they’re covered with soil or crowded by other plants, they’ll rot. Irises may benefit from shallow mulching in the spring. Irises are deer-resistant and drought-tolerant. However, they are susceptible to borers, so check the rhizomes yearly for holes, discarding any infested ones.

The correct time to prune your flowering shrubs depends on when they flower. A rule of thumb is to prune spring-blooming shrubs soon after they finish flowering because most bloom on old wood, which means they set next year’s flower buds shortly after they finish blooming this year. Those that bloom in summer and fall usually bloom on new wood and can be pruned in late winter or very early spring.

Azaleas bloom on old wood. If you wait too long to prune them, you will remove many of next year’s blooms when you prune, especially if you shear azaleas.

It is desirable, with the large flowered rhododendrons, to remove the withered flower clusters after the blooming season. This is fairly easily done because the central axis of the cluster, usually called a truss, will break free from the plant with a push of the thumb, pushing on the side, or can be cut off with a hand pruner.

Dead-heading is usually done to make the bush look more attractive, to reduce the prevalence of fungus, and to prevent a heavy set of seed. If it is not possible to remove the old flowers, it is usually not too detrimental, but flowering the next year may be reduced. When dead-heading, care should be taken to not damage the growth buds or new shoots which are located just below the flower cluster.

Lilacs, like apples, tend to flower lightly one year then overdo the next. It’s just the way they are. If you notice, they tend to sucker heavily at the base and form crowded colonies of slender trunks. Pruning on lilacs is done in midsummer. After flowering, the first thing to do is to thin out the older stems right at the bottom, one-third each successive year. This improves air circulation.

Lilacs are prone to powdery mildew in a sultry, high humidity year. It also jump-starts vigorous new growth. Also remove damaged or crossing branches. If possible, prune old blooms, being careful not to damage newly-formed flower buds (these are next year’s flowers). Lilacs are shallow-rooted and benefit from mulch to conserve moisture. Because they prefer neutral to alkaline soil, a dusting of wood-ashes once in a while is appreciated.

“Growing Green” is contributed by Lehigh County Extension Office Staff and Master Gardeners. Information: Lehigh County Extension Office, 610-391-9840; Northampton County Extension Office, 610-746-1970.