Arborvitae trees, which provide privacy, can be susceptible to leaf...

Arborvitae trees, which provide privacy, can be susceptible to leaf miners and don’t like wet foliage. Credit: Handout

DEAR JESSICA: As you know, arborvitaes are one of the most common hedges on Long Island. I have had trouble finding any comprehensive info on them, such as fertilizing, disease prevention and pruning. Thanks for any light you can shed. --Vivian D'Amore, Massapequa Park

DEAR VIVIAN: You are correct about the abundance of arborvitaes in our area. They began growing in popularity in the 1990s, when the woolly adelgid began wreaking havoc on hemlocks, one of the most popular evergreens of that time. Hemlocks were under attack because of their prevalence; there were so many around that the insects simply pulled a chair up to the buffet. Now, unfortunately, arborvitaes are on the menu, so the arborvitae leaf miner has moved in. Not nearly as widespread (yet, anyway) as the woolly adelgid, the leaf miner is a caterpillar that develops into a small tan moth that lays its eggs between leaf scales. Its larvae overwinter in plant tissue and then tunnel into the foliage for feeding, repeating the cycle, resulting in brown leaf tips, but you likely wouldn't notice any damage until the next winter or early spring. The good news is that affected trees can make a really nice comeback with the help of a systemic insecticide containing either acephate or spinosad as long as at least 20 percent of its foliage has survived the attack. To help prevent an infestation, avoid planting only arborvitaes. It's best to avoid planting more than 5 percent of any one species.

Arborvitaes also can be susceptible to several different fungal diseases, so it's important to space them sufficiently to allow for air circulation through and around each tree and to avoid wetting the foliage with sprinklers. Direct water at the roots instead. There can be other problems, as well, but ensuring your plants are healthy will go a long way toward protecting them.

Expect arborvitae, which tend to be slow-growing, to reach 12 to 15 feet in height with a 3- to 6-foot spread at maturity. The trees love full sun but can manage well in partial shade and are relatively drought-tolerant once established.

The best time to prune them is in late winter, before new growth begins, but this also can be done in early fall, after the season's growth has had time to mature. When trimming, never "top off" trees by pruning the main trunk or branch at the center of the tree's top. Instead, trim each side branch back individually, taking care to maintain an overall shape that is wider at the base than at the top, and to avoid cutting into the bare, needleless portion of any branches. If you do, they'll be bare forever.

You can fertilize once annually during spring with a 27-9-9 or similar high-nitrogen product. Just don't apply more than 2 to 4 pounds per 100 square feet. If they're planted in or near a lawn that you're fertilizing, that could be sufficient.

DEAR JESSICA: We have had a recurring problem with what we think are wild onions in our garden. We have been in this house for 16 years, and they keep coming back. We sifted the soil many times and thought we got them all. We even put down weed cloth and they are now popping through. What can we do? They are especially under our big locust tree, so we are afraid that if we use anything too strong it may kill the tree. Please help. -- Arlene Hofler, North Valley Stream

DEAR ARLENE: You aren't alone. I hear from readers concerned about wild onions every year around this time; it's a common problem in Long Island lawns.

The most effective means of eradicating them, unfortunately, is by hand with a trowel because their small underground bulbs must be completely removed to ensure the weed won't return.

If digging is impractical, because the infestation is massive or the task too laborious, chemical herbicides can be used. Getting a product that contains a "spreader sticker" ingredient is essential, as the plant's glossy stems will repel the chemical without it. Be sure to read the label and follow the instructions precisely.

To increase absorption and, therefore, effectiveness, bruise the plants before spraying by stepping on them. That will open up a wound through which the herbicide can enter.

Be aware that products such as Roundup Weed & Grass Killer will also harm surrounding plants, including your tree, so you'll have to apply it to each individual weed with a small sponge. Steel wool would injure the plant and apply the product in one step. Be sure to wear rubber gloves if you do this.

You'll likely have to repeat the process in November and again next year, as not all the bulbs will be active at once. The ones that are dormant this season will need attention as they spring to life down the road.

DEAR JESSICA: Happy spring! I was wondering about the best time to plant dahlia bulbs now that it is spring. I took them out of the ground last fall and they have been in a cool, dry place in our basement. Also, I recently pruned down my rosebushes. When is the best time to put rose fertilizer down? --Kerrie Huser, Mount Sinai

DEAR KERRIE: Plant your dahlias outside around Memorial Day. The best time to fertilize your roses is when there are 3-4 inches of new growth. Apply about 3/4 cup of a 5-10-5 or 10-10-10 product around the perimeter of each bush, but don't let the fertilizer come into contact with the plant. Scratch it in and then water thoroughly.

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