If your hemlock tree needles turn yellow, it usually means there is an environmental factor causing the tree stress. Being able to correctly diagnose the cause of the discoloration is the first step towards effective treatment.
The most common causes of yellowing hemlock tree needles are overwatering, underwatering, winter injury and insufficient light.
In this article, we will discuss the different causes of yellowing hemlock needles and how to remedy each one.
Why are my hemlock tree’s needles turning yellow?
One of the most common reasons hemlock needles turn yellow is overwatering. When the roots of the tree are flooded or waterlogged they will become damaged and will eventually die due to a lack of oxygen.
The most vulnerable feeder roots are the first to become damaged in this situation. When these roots are damaged, they will no longer be able to effectively absorb water and nutrients for the entire plant.
Hemlock trees are more susceptible when they are in marginal or wet sites such as along the street, or places where the soil has a high clay content that does not drain well. Most trees – and not just hemlock trees – have a hard time growing in soggy soil, and can even die after just a couple of days if the flooding occurs during the growing season.
It is quite difficult to identify an overwatering problem in its early stages, especially when the damage is gradual. The rate of the damage depends on the severity of the flooding as well as the age of the plant. Visible symptoms on the tree are usually only really evident after considerable damage has occurred to the roots.
Most symptoms do not present themselves until the summer when increased water demands manifest symptoms on the tree. The damaged roots will no longer be able to cater to the needs of the whole tree, so the tree will lose vigor and its health will decline over the next few years until it dies.
Even if you think your 15-year old tree looks healthy, it could be growing in heavy soil or in a poor site.
Newly transplanted hemlock trees or seedlings are more sensitive to the effects of overwatering.
A tree going through an active growth stage is also more vulnerable than a tree that is dormant.
Aside from root damage caused by overwatering, the dead roots will be susceptible to opportunistic pathogens and pests that can cause root rot. Trees with root rot will have a more rapid decline in health.
You can minimize the possibility of overwatering your hemlock tree by checking the site prior to planting and following proper planting techniques. If you suspect overwatering in your younger plants, you can avoid opportunistic pathogens and pests by removing any rotten roots.
Underwatering
Another reason your hemlock tree is turning yellow may be underwatering. The feeder roots are also the first to become damaged in this situation and, because these roots are compromised, this will result in a water deficit in your tree.
The symptoms of underwatering vary and most may not become evident until after a year of the roots getting damaged. Just like with overwatering, seedlings and newly transplanted hemlock trees are much more easily affected by underwatering than mature, established trees. This is because these young and vulnerable plants have roots that are closer to the surface where the soil is the first to dry out.
It will take at least two years after planting before your hemlock tree is established, but even then it is not immune to the effects of underwatering. This is especially true of trees planted in pockets of soil on ledges, as well as those in sandy soils. An improperly planted hemlock tree is also susceptible to the effects underwatering.
If you live in an area where hemlock trees are native, you might have less to worry about since the plant has adapted to the climate. However, they can still be affected in cases of severe drought.
As with overwatering, damaged roots due to underwatering are also prone to opportunistic pathogens.
You can avoid underwatering by making sure you water the soil when you see that it is drying out. These trees need one inch of water every week, and they like to get this as a slow, deep soak. Choose an appropriate site to plant your tree and follow proper planting techniques. You can add mulch over the topsoil to better retain moisture and keep the soil moist for longer periods between watering. Remove any damaged or dead tissue to avoid opportunistic pathogens and pests.
Winter injury
If your tree is growing in a wind-swept location, it can get winter injury or dry out due to factors that cause a water deficit. The water in the needles will evaporate over warm or windy days, even during the winter, and if the roots are already damaged due to a water deficiency, the tree cannot replace this evaporated water. The cold soil also makes it harder for the tree to obtain sufficient water.
The damage from winter injury can usually be seen on one side of the tree or on one branch. This is the side that faces the wind, and you will see that the needles are yellow or brown. The symptoms are often unseen until the following spring or summer. A tree with winter injury is also more vulnerable to opportunistic pathogens and pests.
Unfortunately, there is no cure for winter injury, but you can minimize the effects by choosing an appropriate planting site and deep-watering the soil around the tree before the ground freezes in the fall. Prune any dead or weakened branches to avoid secondary problems. You can also put up physical barriers to help minimize the drying effects of the winds.
Needle drop
Low light conditions can also result in the yellowing of the needles, and can cause a slow decline in the general health of even established hemlock trees.
The first symptoms of insufficient light can be seen in parts of the plant where exposure to light is limited. You will see that the needles are turning yellow and dropping. Eventually there is branch dieback and the tree will lose vigor.
You can minimize needle drop by following a regular routine of pruning and general care.
Conclusion
Hemlock needles usually turn yellow due to an environmental factor that is causing the tree stress. The first step in treating this discoloration is to correctly identify the cause of the stress.
The most common causes of yellowing hemlock tree needles are overwatering, underwatering, winter injury and insufficient light.
Image: istockphoto.com / mdurajczyk