Gardening by Month - December
Monthly Tips and Tasks

Category
 
Week
 
Activity 
  1 2 3 4  
 Houseplants x x x x Water houseplants with tepid water. Cold tap water may shock plants.
  x x x x Be sure newly purchased indoor plants are well protected for the trip home. Exposure to icy temperatures for even a few moments may cause injury.
  x x x x Overwintering geraniums like bright light and cool temperatures. Keep soils on the dry side.
  x x x x On cold nights, move houseplants back from icy windows to prevent chilling injury.
    x x x Holiday poinsettia plants do best with sun for at least half the day and night temperatures in the 50's or 60's. Keep plants away from drafts, registers and radiators and let the soil should dry only slightly between thorough waterings. Be sure to punch holes in decorative foil wraps to prevent soggy soil conditions.
  x x x x Hairspray works well to keep seed heads and dried flowers intact on wreaths and arrangements.
  x x x x If you plan to have a live Christmas tree, dig the planting hole before the ground freezes. Mulch and cover the backfill soil and the planting hole to keep them dry and unfrozen. When you get the tree, store it outdoors in a cool, shady, windless area until the last minute and mulch the roots to prevent cold injury. Don't allow the tree's roots to become dry and spray the needles with an anti-transpirant to reduce moisture loss.  Set the tree up in your coolest room. Don't keep the tree indoors for more than one week and plant outdoors promptly.
  x x x x Be sure the root zones of azaleas and rhododendrons are thoroughly mulched. Any organic material will do, but mulches made from oak leaves, shredded oak bark, or pine needles are preferred.
  x x x   Christmas trees hold needles longer if you make a clean, fresh cut at the base and always keep the trunk standing in water.
  x x x   Only female holly trees bear the colorful berries. There must be a male tree growing nearby for pollination, if fruits are desired.
  x x x   Hollies may be trimmed now and the prunings used in holiday decorations.
 Miscellaneous     x       Apply mulches to bulbs, perennials and other small plants once the ground freezes.
  x       All power equipment should be winterized before storage. Change the oil and lubricate moving parts. Either drain fuel systems or mix a gas stabilizing additive into the tank.
  x       Clean and oil all garden hand tools before storing for winter.
  x       If you feed rabbits corn or alfalfa, they may leave fruit tree bark unharmed.
December Pests and Problems


Continue to inspect indoor plants closely for insect pests such as aphids, spider mites, scale, and whitefly.
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Aphids - Indoors
Aphids (Hemiptera) are a common problem on indoor plants. Sticky honeydew on leaves is a common first sign that they are present. 
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Spider Mites - Indoors
Close-up of two-spotted spider mite (Acari) on angel's trumpet (Brugmansia); includes adults, immatures and eggs 
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Scale Insects - Indoors
Close-up of scale (Hemiptera) on frond of kentia palm (Howia

Also check for whitefly, mealybugs and thrips. Treat if necessary.
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Whitefly - Indoors
Whitefly (Hemiptera) on underside of fuchsia leaf (Fuchsia
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Mealybug - Indoors
Mealybug (Hemiptera) colony on coleus (Solenostemon
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Thrips - Indoors
Close-up of cuban laurel thrips (Thysanoptera) on leaves of a weeping fig (Ficus benjamina

Check trees and shrubs in your yard for “volcano” mulching - mulch piled high around the base or trunk of the plant. If found, pull back the mulch from the trunk and create a thin 2-3" deep “donut” over the root zone, but not touching the trunk. A heavy layer of mulch around the base can lead to girdling roots as well as foster insect, disease, and rodent problems.
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Girdling Roots
Girdling root on maple (Acer
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Girdling Roots
Close-up of mulch volcano 
 

Mulch roses after a hard killing frost where temperatures drop into the upper teens.
 
 
 

Remove and dispose of the foliage of plants such as roses, peonies, iris, daylilies, apples, horsechestnut, which are subject to annual fungal leaf diseases.
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Peony Blotch
Peony blotch on peony (Paeonia
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Black Spot of Rose
Yellowing rose (Rosa) leaves with black spots are characteristic of black spot of rose 
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Apple Scab
Yellowing leaves and spots caused by apple scab on crabapple (Malus

Scout for, remove, and dispose of bagworms and cedar-apple rust galls on junipers anytime now until spring. Also check arborvitae, spruce, crabapples, and oaks for bagworms.
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Bagworms
Bagworm (Lepidoptera) on lilyturf (Liriope
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Cedar-Apple Rust
Galls of cedar-apple rust on juniper (Juniperus
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Cedar-Apple Rust
Cedar apple rust gall on eastern red cedar (Juniperus virginiana

Stop fertilizing and reduce watering of indoor plants during winter. Uneven watering can result in oedema. Keeping soil too wet can also lead to an infestation of fungus gnats.
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Oedema
Close-up of ruptured cells on underside of indoor plant leaf caused by oedema 
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Fungus Gnats
Fungus gnat (Diptera) on fruit of ponderosa lemon (Citrus
 

Heavy snow on trees and shrubs, especially evergreens can lead to breakage. Brush off snow carefully before it melts and refreezes and becomes difficult to remove. Allow ice to melt off naturally. Ice causes branches to become brittle and easily broken.
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Winter Injury: Frost Injury, Ice Damage
Snow buildup on a Japanese falsecypress (Chamaecyparis
 
 

Diseased branches in apples, pears, pyracantha and other plants in the rose family that were killed by fireblight can be safely removed in mid-winter. If not removed in winter, wait until dry weather in summer. Avoid pruning plants susceptible to fireblight during spring when the bacteria can easily enter cuts resulting from pruning.
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Fireblight
Tip dieback on crabapple (Malus) caused by fireblight 
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Fireblight
Fireblight canker and twig dieback on crabapple (Malus
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Fireblight
Dead shoot on apple (Malus) showing droplets of the bacterial ooze caused by fireblight 

If overwintering dormant perennials in a garage or basement monitor the temperature and moisture levels closely to avoid freezing, drying out or rotting due to overly wet, cold soil.