The Ornamental Garden in December

This monthly list of tips and tasks will cover three areas: outdoor garden care, holiday tips, and houseplant care.

OUTDOOR GARDEN CARE

Take a walk around your landscape making notes about what you would like to add or subtract from your gardens next season. Make notes on thinning or transplanting perennials that have become overgrown.
Consider repairing or adding hardscape projects.
Look for areas in your landscape where water collects and make a note of the location and the degree of wetness before and after rain or snow. See this article for ideas on how to address drainage issues both large and small. Some approaches may be as simple as aerating, adding soil gradually as a top dressing, or planting water tolerant plants. Solutions to larger drainage problems might include regrading or redirecting runoff safely to a suitable area. This can be done using swales, French drains, catch basins or downspout extensions.
Be aware of any plants that have already begun to heave with freezing and thawing; place them gently back into the soil to protect their roots.
Remove spent foliage from perennials that are prone to fungal diseases such as Pulmonara (lungwort), peony, and hosta.
Lay a 2-3” layer of mulch or leaves on perennial beds and shrubs after a hard freeze or when the soil temperature reaches 32°F.
Pull mulch back from tree trunks.
Loosen or removed matted layers of thick leaves.
Clean garden tools with steel wool or a stiff wire brush, wipe with a damp cloth, dry, and apply an oil such as cooking oil, boiled linseed oil, tung oil, or WD 40.
Shut off and drain outdoor water pipes and irrigation systems.
Put away concrete bird baths, planters, rain gauges and other outdoor ornaments that might collect water and break in freezing temperatures.
Plant spring-flowering bulbs before the ground freezes; although December may be late for bulb planting, it’s better to plant these bulbs now rather than waiting for spring. Mulch after planting to extend conditions for root growth.
Provide food and water for the birds; be sure to change and clean water sources, clean bird feeders on a regular basis to avoid spreading disease.
See this article from the National Wildlife Federation on how to provide water for birds in winter.
Keep watering recently-planted trees and shrubs. Long, dry periods during fall and winter can cause injury or death to plant root systems.
Continue to plant deciduous trees before the ground freezes. Your shovel will be unable to penetrate the soil if it is frozen. Avoid transplanting shrubs and trees on sunny or windy days, which can expose the roots to light and drying winds, stressing the plant. Mulch the area heavily to keep the ground thawed so roots can become established. See the November 2021 issue of The Garden Shed for helpful tips and a list of informational sources on planting trees.

INVASIVE PLANT CONTROL

With colder weather and holiday distractions, we may forget that invasive enemies are still at work. See this article from the Blue Ridge PRISM (Partnership for Regional Invasive Species Management) on identifying and treating invasive plants in winter.

HOLIDAY TIPS

TREES

Live tree or artificial? For some families, this is a question revisited each season. See this past article from The Garden Shed for key points to consider.

For tips on the selection and care of Christmas trees, plus a description of types of trees, see this article from NC State Extension.

Some people like to plant a living Christmas tree, either potted or balled-and-burlapped, into their landscape after the holiday season. For tips on selection and care of trees purchased for planting outside, see this article from NC State Extension.

General holiday tree care tips:

If a tree has been stored out of water for three or more days, it is a good idea to make a fresh cut on the trunk prior to display. Remove a disk about half-inch thick to provide a clean fresh surface for water uptake.
Use a tree stand with a large reservoir for water. The water level in the stand should be checked and replenished daily. If the tree stand is allowed to dry out, the tree may not readily take up water again. When trees dry out, they should be removed before they create a fire hazard.
Do not turn on Christmas tree lights if you think your tree has become dry.
Do not use any Christmas tree preservatives or other additives in the water. University research has repeatedly shown that water additives are not needed and, in many cases, aggravate foliage drying and needle loss. This is true for both commercial products and home remedies such as sugar, aspirin, and even VODKA!
The tree should be set up away from sources of heat such as heat vents, space heaters, or fireplaces. Even setting a tree up in front of a south-facing window can dry a tree out if it spends the day in full sun and is watered inadequately.

HOLIDAY GREENS AND BOXWOOD BLIGHT

Be very mindful of the threat of boxwood blight when bringing any boxwood greens or boxwood wreaths into your home. For more information on boxwood blight, see “Boxwood Blight Alert” in the November 2018 issue of The Garden Shed, as well as information from the Boxwood Blight Task Force. Don’t unwittingly contribute to the spread of boxwood blight by carelessly tossing out boxwood greens used for holiday decorations. Err on the side of caution–double-bag boxwood greens securely and dispose of them in the landfill. Do not add them to the compost pile. Buy boxwood greens from licensed vendors that can provide a copy of their Boxwood Blight Cleanliness Program Agreement.

HOLIDAY PLANTS

Poinsettia is the plant most frequently identified with holiday decorating. Give this plant sun for at least ½ day; ideal night temperature is in the 50s to 60s. Keep the plant away from drafts, heat registers, and fireplaces. Soil should dry only slightly between thorough watering. Be sure to poke drainage holes in decorative foil or remove foil to keep the soil from becoming soggy.

After buying poinsettias, protect them from cold on the way home. Exposure to icy temperatures for even a few moments may cause injury.

Holiday cacti, including Thanksgiving and Christmas cacti, are other popular plants for the holidays. Preparation should begin 6-8 weeks before desired flowers. These conditions are recommended for best flowering:

a cool night temperature of 50-55°F
13 hours of uninterrupted darkness (if the temperature is 55-70°F)
15 hours of darkness (if the temperature is above 70°F)

For more information on holiday cacti, see this past article from The Garden Shed.

Cyclamen is another holiday plant option. The houseplant, or florist’s cyclamen (Cyclamen persicum), is a tuberous potted plant that flowers during the winter months. It has lovely, heart-shaped green foliage mottled with silver. Blooms come in shades of white, pink, rose, purple, and red. Cyclamen prefers cool temperatures and bright indirect light. Ideal daytime temperatures are 60-65°F with night temperatures around 50°F. Cyclamen prefers to be kept moist, but not soggy. Water when the potting medium feels dry to the touch, and always water along the edge of the pot, or from below, to avoid causing the tuber to rot. Unlike many other houseplants in winter, cyclamen can benefit from periodic light fertilization.

Although they can be considered persnickety, the photos below show the same cyclamen that has bloomed continuously from 12/20 through 12/21 (and I hope beyond).

HOUSEPLANTS

The growth rate of most houseplants will slow during the cooler temperatures and shorter days of the winter months. Reduce both fertilization and water until late April or May when new growth resumes.
As nights become colder, back houseplants away from windows to prevent chilling injury.
Excessive moisture from watering and humidity can cause oedema. This condition is caused by roots taking up more water than foliage can transpire. Leaf cells rupture, particularly on the undersides, leading to water-soaked patches that look corky and unsightly. This condition is easily curable if caught early. Reduce watering and humidity. Do not remove unsightly leaves since this can worsen the condition. Plants that are especially susceptible include cacti, succulents, orchids, and semi-succulents such as Peperomia.
Check for pests such as aphids, spider mites, scale, whitefly, mealybugs, and thrips. See this article from the Missouri Botanical Garden for photos, ID, and treatment.
Group plants together to increase humidity levels but leave sufficient space for air circulation.
If grouping plants together, be aware that this may increase the potential for insects infecting neighboring plants. Monitor frequently.
When you are away, place houseplants on trays or saucers of pebbles with water below the top surface of the pebbles. This technique can be used effectively for about 1 week.
Pebble trays can also be used to increase humidity during the winter heating months.
Pebble trays Photo: Susan Martin