Thuja Green Giant Tree Problems

Thuja ‘Green Giant’ is susceptible to very few pest or disease problems. Most gardeners will never have any issues with their ‘Green Giants’. But if you do encounter pests and diseases of Thuja Green Giant, here is how to solve them in our list of Thuja Green Giant Tree Problems.

Thuja Green Giant arborvitae shrub is one of our most popular ornamental trees for hedges, privacy screens, and windbreaks. It is a fast growing evergreen shrub, adaptable to a wide range of soil conditions, and grows fast. With a pyramidal shape, it can grow up to 20-40 feet tall and up to 20 feet wide at maturity. Thuja has an extremely fast growth rate of 3-5 feet per year once established in full sun or partial shade.

It is normally very disease resistant with the proper Green Giant Arborvitae care and preventative measures. The arborvitae foliage should stay green all year round and will be the first indicator of something wrong if foliage color changes.

The emerald green Thuja plicata is grown in the United States department of agriculture plant hardiness zones 5-9. So be sure you are growing in the correct zones first and foremost…

Here are some normal Arborvitae diseases and how to fix them:

  • Arborvitae Turning Brown Leaves

If some branch tips are turning brown on your newly planted ‘Green Giants’, it might be because they aren’t getting enough water .

Remember, newly planted trees and shrubs need lots more water than established plantings. For the first few months, you should water your trees every day or two that it doesn’t rain. At least a gallon of water per week is adequate. After a few months of growth, when their root systems have penetrated into the soil around them, the rich Green Giant evergreens should be fine with just an inch of rain or supplemental water per week or two. They are moderately drought tolerant, as well as tolerant to heat and humidity.
  • Arborvitae Turning Yellow

If, after a few weeks in the ground, your ‘Green Giants’ appear a little yellowish rather than overall bright green, they could be suffering from nutrient deficiency. We did fertilize when we planted them, but now is the time to begin fertilizing again.

Use a slow release balanced fertilizer labeled for large evergreen trees and shrubs and follow label directions. We prefer the more expensive slow-release or organic fertilizers since they don’t need to be reapplied as often. Usually, only once a year is enough.

Thuja ‘Green Giant’ can sometimes turn a little yellowish in cold winters, especially if it’s in an exposed, windy location. This is normal and the Arborvitae tree should be its dark green self again when spring returns.
  • BugsBagworms on Arborvitae

If you see little two-inch long cones of interwoven dry needles hanging on branch tips, these are probably the young larvae homes of bagworms. Males emerge when the eggs hatch in early June.

They make the little stick-tents out of silk they produce and pieces of the needles and plant foliage that they eat. Open one up and see the green caterpillar inside. These will come out at night and eat a few needles on your ‘Green Giant’, but rarely are there enough of them to damage the tree. Squish them or plop into a bucket of soapy water. Be sure to check nearby trees (even deciduous trees of other species may be affected).

If you think the infestation is serious enough to stunt the tree’s growth, you can spray with a commercial formulation of Bt. Bt is short for Bacillus thuringiensis, a naturally occurring bacteria that kills caterpillars when they eat leaves or needles that have been sprayed with it. Bt is one of the safest natural pesticides for controlling caterpillar pests on plants and is accepted as organic. Only insects that eat the sprayed foliage are killed. This will work for spider mites too.
  • Sooty Mold on Evergreens

If you notice a black powderlike coating on twigs or needles, it is probably sooty mold. This is a harmless fungal pathogen that grows on the excretions of scale insects which look like little brown bumps on twigs and needles. Scale insects feed on sap they suck from small twigs and stems. Thuja ‘Green Giant’ is so vigorous, and grows so fast, that scale insects are almost never a problem.

However, if your ‘Green Giant’ is growing in poor sandy soil, or soggy soil, or not getting enough water, its growth could be stunted by a large infestation of scale insects. Usually, all you need to do is prune and dispose of infested twigs.

You can rub the little turtle-like critters off by hand. You can spray or dab them with alcohol or insecticidal soap or horticultural oil. For very severe infestations, you can use a commercial pesticide containing azadirachtin, which will kill scale insects, not harm honeybees, and is approved as organic.
  • Arborvitae Root Rot

If your Thuja ‘Green Giant’ is growing poorly, turning to reddish brown foliage, and seems to be dying, it could be root rot caused by soggy or saturated soil conditions. Let the soil line get a little dry between waterings.

Water in the mornings so the soil has time to dry out during the day. Check to make sure you don’t have a leaky water pipe or irrigation water nearby. If the soil naturally stays wet all the time, my Green Giant will not survive there. The Thuja prefers well drained soil. If you have clay soil you can also have your soil tested for water content. Susceptibility to phytophthora root rot is a common side effect of wet soils. It is a fungal disease that can spread to infect other plants and is caused by excess water in the roots systems.

  • Cypress Tip Moth Miners:

If you notice the green foliage of you Green Giant’ turning yellow in early winter, then brown in early spring before turning nice and green again, it could be due to cypress tip miners, the larvae of a little whitish moth.

These caterpillars are green or yellow and eat needles on twig tips. Shake a suspected branch in spring to see if moths fly off. Remove and dispose of infected branches. Serious infestations are rare, but can be treated with Bt in winter or early spring.

  • Arborvitae Canker
If you have wilted twigs with yellow or brown needles, sunken lesions on larger branches, and some of them actually dying, your tree may have canker, an incurable fungal disease.

Remove dead and dying branches as soon as you see them. If the cankers are on the main trunk, it is too late to save the tree.
One of the things we love most about Thuja ‘Green Giant’ is its vigor and near immunity to most diseases and pests!

Check out our Thuja Growing Guide for more information on growing this giant.