This month’s Tree Tip was written and researched by Britt Hubbard, Director of Plant Health Care for Woodland Tree Service, and ISA Certified Arborist #SO-5792A. He may be contacted directly via email with any questions concerning this article or any tree related issues.
Winter is considered a dormant season for trees and plants in our area. However, calling this the dormant season leads to a misconception about plants. Although leaves have fallen from many plants and growth has slowed, trees and shrubs are still carrying out life processes, taking up nutrients from the soil, growing roots, and putting on buds for the spring season. Insect pests are also prevalent during the winter. It may be the ‘dormant season’, but it is an important time to be active with plant health care.
Oak Stem Galls
There are numerous insects that create galls on leaves, stems, buds, and twigs. The majority of these do not damage the tree except for disfiguring the affected parts. Once formed and developed, there is little stress on the plants. However, Oak stem galls (Gouty Oak Gall and Horned Oak Gall) are not only unsightly; they interfere with life processes and affect plant health. They disrupt the transfer of nutrients and carbohydrates between the leaves and rest of the tree, reducing vigor and growth. An oak infested with oak stem gall is highly unsightly, and severe infestations can result in damage or death of the tree.
Small non-stinging wasps form the Gouty Oak Gall and Horned Oak Gall. The wasps lay eggs in the newly formed stems in spring and incubate inside the golf ball sized gall. After two or more years the wasps emerge, mate, and start the cycle over again, often in the same tree. Eventually, the empty gall and a portion of the stem will fall out of the tree. To stop the cycle, control treatments must be applied in the ‘dormant season’ so new stem growth will not be susceptible to wasp attack in the spring.
Each year control treatments are applied, new galls will not be formed that year. Over time, as old galls fall out of the tree, the canopy looks more aesthetically pleasing and the tree is healthier and does not lose leaf area due to gall issues or have disruptions between the leaf and tree. Fewer disruptions mean more carbohydrates and energy for the tree; increasing tree vigor and health.
Scale on Trees and Shrubs
Scale insects are often inconspicuous pests of many evergreen and deciduous plants. They can occur on leaves, twigs, branches or trunks on trees (Oaks, Elms, Maples, and more) and shrubs (Holly, Boxwood, Euonymus, and more). Their small size and general lack of mobility make them difficult to notice by the casual observer. Scales derive their name from the shell-like, protective covering they form over themselves.
Scale insects cause damage by removing vital plant fluids from their hosts using their sucking mouth parts. Leaf and needle stunting and yellowing, twig and branch dieback, as well as plant death are possible depending on population levels. In some instances, scales weaken plants making them susceptible to damage from secondary pests such as borers or environmental extremes, which may ultimately kill the plant.
Scales can also create nuisance problems by producing a sticky, sweet substance called honeydew, which they secrete while feeding. The stickiness and associated black sooty mold that grows on the honeydew can be an annoyance if cars, patio furniture, decks, etc., are underneath scale-infested trees.
Winter is a great time to apply control treatments for scale. Horticultural oils are applied to reduce overwintering scale and fewer leaves ensure better spray coverage on the tree or shrub. Treatments applied in the winter will reduce scale populations throughout the growing season.
Aphids and Whiteflies on Shrubs
Aphids and Whiteflies are closely related insects pests that attack the underside of leaves, and succulent new growth on stems and buds on Holly shrubs and other landscape shrubs. 
Aphids are green and orange insects that readily move around the plant. Whiteflies are small and white often covered by a cottony/waxy material and do not move much from the underside of leaves. The insects are sucking pests that suck sap from the plant and secrete honeydew on the adjacent leaves and landscape.
The excessive honeydew then creates sooty mold issues just like scale pests. When environmental conditions are right, aphid and whitefly populations can explode creating major problems for shrubs. Affected shrubs have dieback, reduced vigor, stunted leaves, stem and flower growth, and are susceptible to diseases spread by the insects.
Winter is a great time to apply control treatments for aphids and whiteflies. Horticultural oils are applied to reduce overwintering eggs. Treatments applied in the winter reduce populations throughout the growing season.
Spider mites on Trees and Shrubs
Spider mites are another insect pest that are susceptible to winter control treatments. 
Spider mites attack Holly shrubs, Boxwood shrubs, Arborvitae, and other Juniper trees/shrubs. Spider mites are small pests that are barely visible with the unaided eye. When populations are large, they create webbing to protect themselves and their eggs from natural predators. Spider mites suck juices and chlorophyll from leaves damaging leaf tissue. Symptoms of injury include flecking, discoloration (bronzing) and scorching of leaves. Injury can lead to leaf loss and even plant death.
Treating during the ‘dormant season’ will reduce populations and stop damage from occurring to the landscape plants. Affected trees and shrubs should be sprayed with a horticultural oil and miticide to control active and overwintering populations.
Deep Root Fertilization
Although this is the ‘dormant season’, trees and shrubs are still taking up nutrients from the soil to grow roots and prepare for the growing season. Trees that have not had their fall DRF treatment will benefit greatly from a treatment now. DRF ensures the tree has appropriate nutrients (N,P, and K) to carry out life processes which increases the trees ability to compartmentalize decay (reducing decay and dieback) prevent disease (healthy trees are less susceptible to disease) , and fend off insect attacks (healthy trees are less desirable to insect pests).
We want your trees and shrubs to be healthy throughout the whole year! If you need any assistance with your trees, please contact the experts at Woodland Tree Service. Email info@woodlandtree.com or call us at (901) 309-6779.