Garden Detective: Questions about blue spruce, Dracaena and Schefflera

DEAR JESSICA: I have a blue spruce that’s about 20 feet tall, which I planted six years ago. Every year, it has produced beautiful blue branches until last year, when I noticed it has a lot of brown branches. Can you help? — Robert Pultorak, Valley Stream

DEAR ROBERT: Healthy blue spruce trees tend to replace their needles every 10 years or so. It’s a little early for your tree to be going through a cyclical needle drop, but you don’t say how old the tree was when you acquired it. During cyclical needle drop, old needles are replaced by fresh, green growth. If needles are browning and dropping without being replaced, however, odds are the tree is suffering from a fungal disease called Rhizosphaera needle cast.

Visible damage begins at the bottom of infected trees and travels upward, with inner needles turning brown in late winter and dropping in summer, leaving trees looking sparse. To confirm the diagnosis, use a magnifying glass to inspect affected needles for lines of tiny black dots. If they are visible, there is nothing you can do to cure the diseased plant parts, but you must take action to prevent spread to the rest of the tree (and potentially nearby trees).

Remove affected branches, and rake up fallen needles from around the tree, discarding both in the trash. In spring this year and next, when new growth is a half-inch long, apply a fungicide labeled as effective against the disease, and repeat every three or four weeks until autumn. Then fortify the tree by applying a high-nitrogen fertilizer, such as Arbor Green PRO, in May or June.

Fungi thrive during prolonged wet or humid weather, and on trees planted in shade and/or too close together. To help prevent a reoccurrence, avoid overhead irrigation; instead water deeply, with a soaker hose encircled over roots, during dry spells. 

If you don’t spot the disease’s telltale black dots, other culprits, such as spider mites or scale, could be at work. In that case, clip and bring a few affected branches to the Cornell Cooperative Extension plant diagnostic clinic nearest you (for information, call 516-565-5265 in Nassau; 631-591-2314 in Suffolk) or call a certified arborist for a diagnosis and treatment plan.

DEAR JESSICA: I’ve had this houseplant for a long time. Can you tell me why its leaves are turning yellow and brown? Also, it is getting very tall and I see new growth at the bottom. Should it be cut? — Linda Cortes, Lake Grove

DEAR LINDA: Your Dracaena’s foliage is likely turning yellow because of a watering problem. One possibility is overwatering; allow the soil to dry completely between waterings.

In addition, these plants (and many others with long, slender leaves) are particularly sensitive to fluoride, so if your community adds the mineral to its water supply, let water sit it a container overnight before applying to plants; this will drastically reduce or eliminate the fluoride. Perlite, a component of many potting mixes, also contains fluoride, as do fertilizers that contain superphosphate, so avoid both. 

And you should be watering only with room-temperature or tepid water; too-cold water could adversely affect foliage.

Dracaenas prefer daytime temperatures between 60 and 70 degrees and nighttime temperatures between 50 and 60. Keep plants away from drafty windows or heat sources.

You can trim the top of your plant if it’s grazing the ceiling, but do so strategically, knowing that side shoots will grow from the pruned branches. The best time to trim is after the plant resumes active growing in spring.

DEAR JESSICA: We inherited this plant from my husband’s sister who passed four years ago. As such, we are very attached to it. However, we are at a total loss as to what kind of plant it is, and how to manage it so that it will thrive for years to come.

This summer I put the plant outside on the deck, and it grew quickly and very unevenly. I repotted it when we brought it back inside in the fall, and it is threatening to take over the bedroom where we now have it under the window.

Any advice you could give us as to what it is, and how to prune it into a more manageable and attractive shape would be greatly appreciated. — Fran and Steve Zito, Fort Salonga

DEAR FRAN AND STEVE: Your plant is a dwarf umbrella tree of the genus Schefflera, and it appears to be thriving, indeed. Most houseplants placed outdoors for summer will experience a growth spurt if provided the right sun exposure and cultural conditions, as yours obviously has.

Indoors, these plants require bright, indirect light and prefer temperatures between 65 and 85 degrees. They will grow leggy and perhaps flop over with insufficient exposure; they will experience leaf burn if the light is direct.

Water only when the soil has dried out, using water that has been left out in a container overnight, soaking the soil slowly and thoroughly before allowing it to dry out again (yellowing leaves indicate overwatering).

Dwarf umbrella trees are light feeders, so fertilize no more than once a year, in early spring, with ordinary houseplant fertilizer diluted to half strength.

Pruning is easy: Simply trim it to the desired size and shape.