How to control invasive orchids, powdery mildew and diminished blooms on dogwoods

DEAR JESSICA: I have two dogwood trees in my yard, and in the past two springs they have produced very few flowers. It breaks my heart, as I love these trees. They were both Mother’s Day gifts from my family. Is there something I can do to help them and bring them back to their May glory?

 — Kathy Finch, Rockville Centre

DEAR KATHY: Dogwood trees (Cornus) require well-draining, fertile soil and partial shade in order to thrive and bloom best. Because of these needs, they are considered understory trees, which means that in the wild, such as forests, they typically are found growing under larger trees, which provide shade as well as nutrients from hummus in a soil produced from the fallen leaves of their neighbors. They do require some sunlight, but too much or too little certainly would impact blooming.

I see larger trees in the background in your photo and wonder whether they have been cut back recently, which would have increased the amount of sunlight that reaches the dogwoods and could be affecting the number of flowers they are producing. If, on the other hand, you notice the surrounding trees have grown larger and there is now more shade than in the past, consider thinning surrounding trees to allow filtered sunlight. Other sources of dense shade would be an extension added onto the house or another new sun-blocking structure.

Pruning at the wrong time of year — winter or early spring for dogwoods — removes buds that would be flowers.

And another consideration is fertilizer: If the dogwoods are sited in or near a lawn that receives nitrogen fertilizer, it’s likely the nitrogen is forcing the trees to route their energy toward growth at the expense of flower production. When fertilizing nearby lawns, avoid the soil area from the tree’s trunk extending outward under the tips of the farthest-reaching branches, which is referred to as the “drip line” of the tree.

DEAR JESSICA: Over the last two years, our property has become infested with the invasive orchid Epipactis helleborine. What can be done to eliminate these rapidly growing plants?

 — Jay Zuckerman, Mount Sinai

DEAR JAY: The weedy orchid you’re describing actually was introduced intentionally from Europe, with its first New York sighting reported in 1879. It soon became apparent, however, that the pretty landscape plant takes off like a runaway train on this side of the pond through its underground rhizome network, which sends up several plants from each root as it grows and spreads beneath the soil. Unfortunately, control is quite difficult, as leaving behind even a small portion of rhizome will result in more proliferation. In addition, the plant doesn’t respond well to herbicides such as glyphosate, the active ingredient in Roundup.

There are only two things you can do in an attempt to keep this orchid under control: First, dig deeply to remove every last bit of root you can find, taking into account that the fleshy rhizomes can extend more than 4 inches into the soil, and dig again whenever new plants surface. Next, never allow the plants to go to seed; they will germinate and sprout new plants two years later.

DEAR JESSICA: I planted a lilac bush about four years ago, and every year it gets chalky white on the leaves. It finally has buds this year, and I don’t want it turning white. How do I prevent this from happening?

 — Marina King, via email

DEAR MARINA: What you’re seeing is powdery mildew, which is a common affliction of lilacs. The fungus appears as a white or gray powder that coats leaves and stems. Susceptible plants, such as peonies, phlox and hydrangea, in addition to lilacs, should be regularly monitored. Left unchecked, symptoms that include stunting, leaf curl, yellowing or browning of foliage, shortened bloom time and premature flower drop can escalate, making plants unsightly. But the good news is powdery mildew seldom is fatal.

Affected leaves should be removed and disposed of (in the trash) as soon as you spot them, and plants can be treated with horticultural oil or neem oil to control outbreaks (follow package directions carefully). For severe infections, synthetic, sulfur and biological fungicides are available. Good cultural practices should be employed to prevent infection and recurrences. Simple measures such as planting in full sun, allowing adequate spacing of plants to accommodate their mature sizes, pruning to thin out branches or stems to allow more sunlight to reach the middle of the plant, dividing larger plants and removing fallen leaves and plant debris from garden beds can help keep plants healthy and prevent infections from spreading and recurring.