Author: Miri Talabac

Q: We have some welcome respites of very mild to warm weather in the forecast. When can I put some of the plants I overwintered indoors back outside? I’m sure they miss the better light.

Emily Clark Waterson2 Comments
A vaiegated spider plant and some succulents in containers on a balcony.
Some houseplants are tougher than others but most are not ready for full sun or the cold nights of early April. Photo: Anna/Adobe Stock

A: It’s easy to get caught-up in the moment during those lovely spring-preview days and want to liberate your houseplants to bask in the great outdoors. I know I’m antsy to get mine back outside. Try to wait until at least our last frost, though, which for central Maryland averages about Mother’s Day. Some tropical plant species are more sensitive to cold than others, so you may be able to start the acclimation process for the tolerant ones earlier, if you pay attention to overnight temps and bring them back in when needed. In comparison, some may sulk if they’re exposed to temperatures below 60°F. If unsure, err on the side of caution; exposure to no chillier than 55-60°F is a good assumption.

All plants, no matter how sun-loving they are, need time to adjust to the brighter light moving from indoors to outdoors, so they don’t sunburn. Shade outdoors is usually much brighter than direct light indoors, even though to our eyes it doesn’t look that different. Put anything you bring out into shade first, gradually introducing them to greater amounts of direct light (based on their needs) over a few weeks.

Even plants that need bright light inside don’t necessarily want unobstructed full sun outside, even once acclimated. Potted succulents and air plants, for instance, tend to fare well if shielded from the strongest afternoon summer sun by some dappled shade from either trees or shade cloth, or by a passing shadow from a building or fence.

Various potted plants and herbs growing on a balcony with a railing and a large tree in the background
A cozy balcony garden filled with various potted plants and herbs in mid summer. Photo: Emily Clark Waterson, UMD Extension

By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun and Washington Gardener Magazine. Read more by Miri.

Q&A: How can I fix flowering issues with my Christmas cactus?

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Christmas Cactus with a few dark pink blooms near a window.
This Christmas cactus may be struggling to bloom due its location in the home with lower light conditions.
Photo: Ask Extension, UMD HGIC.

Q: This year, I am again having problems with the flowers on my Christmas Cactus. The plant gets an abundance of flower buds, but most of the ones that flower do not flower completely. Many partially flower, but then start drooping before fully blooming and opening. Also, many of the buds fall off before blooming. I am careful not overwater, and the plant is in a window facing the east.

A: It can be difficult to know for sure what is causing the symptoms, but possibilities include low ambient humidity (these cacti are native to jungle environments, where they grow as epiphytes on the trees like orchids do), temperature fluctuations, under-watering, and possibly exposure to ethylene gas, which can come from a group of ripe fruit and leaks in gas appliances.

When watering, make sure the potting mix is thoroughly rehydrated, and excess water freely drains out of the bottom drain holes. To boost humidity, consider using a humidifier (misting foliage will not be effective).

An east-facing window doesn’t give the plant much light (especially this time of year and if the plant isn’t directly in front of the pane), so if you have an option to grow it in a west- or south-facing window, that may help with growth and vigor overall, even if it may not change much with regards to flower bud dropping. In fact, that may need to wait until after the flowering period, as Iowa State notes that “to avoid flower bud drop, do not move the plant during flower bud development,” though it can be moved once flowers are opening. Michigan State mentions a couple of other causes, such as exposing the plant to light that interrupts its required 13 hours of darkness (a seasonal trigger for flowering) and exposing the plant to temperature swings (drafts), particularly below about 50 degrees. The linked page has more details about the darkness period.

You can also refer to a blog post from a few years back for some further reading: Christmas cacti make lovely gifts and decor by Annette Cormany.

A lush Christmas cactus displaying colorful magenta blooms, sitting in a bright window.
Heavy blooms are a hallmark of Christmas cactus and its cousin, the Thanksgiving cactus.
Photo credit: Washington County Master Gardener Wilma Holdway.

By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun and Washington Gardener Magazine. Read more by Miri.

Q&A: Should I Avoid Cut Nandina Berry Decor?

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Two bundles of cut winter greens featuring red berries.
Winterberry branches in mixed greens bunches.
Photo credit:  Miri Talabac

Q:  Are Nandina berries okay to use in holiday decor? I love their bright color and long-lasting berries, but I heard they may be invasive.

A:  I would not use them, since yes, Nandina domestica is invasive in our region. While observations of the species growing in natural areas suggest it hasn’t run as rampant as other invasive shrubs (Multiflora Rose, various honeysuckle species, Autumn Olive, etc.), we should not assume it will stay that way as the climate changes. For example, a decade ago, I didn’t notice Mahonia seedlings to the extent that I do now. They are easy to see in deciduous woodlands in winter, with their evergreen, holly-like leaves on shrubs that are short enough for deer to ravage if they were actually willing to eat them.

For colorful berries in winter holiday arrangements, Winterberry Holly (Ilex verticillata) is a commonly available alternative, and the species is native across the state. (The forms grown for cut branch use are cultivars that produce larger or more prolific berries than wild forms.) The berries should last from when cut branches are first sold for the season (usually early December, at least from local farms) to when you compost the aging arrangement after the new year. As with most cut holiday greens, Winterberry branches hold up best when kept cool and out of direct sunlight.

Other hollies, like Foster Holly (Ilex x attenuata ‘Fosteri’), can also be used for showy branch cuttings, and the hybrid’s leaves are more slender and less prickly than American Holly or other evergreen species. It may be harder to find cut Foster Holly boughs, though, especially since a farm will need to reach high branches to harvest. A red accent to a greens arrangement can also be added via cut stems of Redtwig Dogwood, though overall they’re a darker red color than holly berries. There are native (Cornus stolonifera, which is rare in Maryland) and non-native (Cornus alba and Cornus sanguinea) species and cultivars available on the market, which vary in stem color and brightness. If you grow your own for harvesting, it’s the youngest wood that has the richest bark color; older stems are less showy, which is why older shrubs are often cut back every few years to force more colorful regrowth.

By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun and Washington Gardener Magazine. Read more by Miri.

Q&A: Why Do My Boxwoods Look Off-Color?

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Boxwood leaves that are yellowish and blistered due to Boxwood Leafminer feeding.
Yellowing of boxwood leaves caused by Boxwood Leafminer feeding. Photo credit: Miri Talabac

Q:  I have a low hedge of boxwoods that are looking somewhat orange-tinted and less dense than usual. What might be causing this?

A: Boxwood Leafminer is my primary suspect, and I have recently seen boxwoods looking just like you describe, which had high populations of this common insect pest.

With only one generation per year, this species of fly is currently in its larval (juvenile) stage inside the leaves. Since larvae feed on tissues between the upper and lower leaf layers, you need to peel the leaf apart to find them to confirm their presence. Outwardly, affected leaves will become blistered-looking and have a blotchy yellow or orange-yellow color. Leafminer damage can also cause the shrubs to shed some leaves prematurely, so the branches will have more sparse growth towards the plant’s interior than a healthy shrub would have.

Boxwood Leafminer larvae are tiny orange-yellow legless maggots, only a few millimeters long. However, they are readily visible if you open a leaf and examine it with a hand lens, since the rest of the leaf interior will be light green. They will pause feeding as winter approaches and overwinter in a state of suspended development, resuming more voracious feeding in early spring. Larvae pupate and transform into orange gnat-like adults by late April or early May (depending on temperature trends).

Boxwood leaf tissue has been cut away to reveal Boxwood Leafminer larvae feeding between the upper and lower leaf surfaces. Photo credit: John Davidson, Ph.D., UMD

Some boxwood cultivars have greater resistance to leafminer (as well as to fungal diseases) than others. When the insects are too abundant for their damage to be tolerable, prune out the infested branch tips or, as a last resort, consider using an insecticide to reduce their numbers. Keep in mind, though, that insecticide use can have unintended consequences: it might facilitate an increase in boxwood spider mites (by killing mite predators); harm pollinators visiting the boxwood’s flowers; or harm insect or spider predators wandering its foliage that would be eating pests.

You can learn more about leafminer as well as other common boxwood ailments on Extension’s boxwood diagnostic web page (go.umd.edu/boxwood]).

By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun and Washington Gardener Magazine. Read more by Miri.

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