If you have arborvitae (or other evergreens), you may notice a lot of browning on interior needles this time of year. The good news is that your arborvitae is not dying, it's just time for fall needle loss (most likely anyway - but more on that later). Most conifers drop some amount of old needles in the fall, so if there has been brown on your spruces and pines as well in the past month, they are probably also fine.
Why do healthy evergreens drop some of their needles?
Because they don't need them anymore, or because the needles get too old. As the plants grow, the inner needles get less and less light. At some point, the cost of keeping those needles alive exceeds the benefits of what little food they produce through photosynthesis. Some species will also drop older needles and replace them with new ones. The greatest number of these needles will be dropped in the fall, but sometimes you see healthy needle cast at other times of year.
Why didn't it do this last year?
Not all species go through fall needle cast every year. Spruces in particular tend to hold onto needles longer, while pines drop them more regularly. It may also be that needles did drop last year, just not as many. If a tree or shrub has had a stressful year, it will probably drop more needles. This still doesn't mean that your plant is dying. If it continues to lose needles into the spring, it's time to be concerned that serious stress or a disease is involved.
How can you I be sure I'm dealing with normal fall needle cast?
There are many reasons an evergreen may be turning brown - and most of them aren't good - so let's set down four guidelines for how to tell if all is well with your tree or shrub.
Only the interior needles are affected. If you are seeing brown tips, or entire brown branches, then you may be dealing with a problem. The possible explanations range widely, so I won't get into that much here. Pines are an exception to the rule. They tend to thin out needles from all over the branch rather than dropping entire sections from the inside out. White pines are especially guilty of heavy annual fall needle drop.
Browning is not limited to one side of the plant (unless one side receives more shade). Needle drop should be fairly uniform across the whole plant. Shaded sides may have more obvious browning since evergreens grow less densely in the shade, so it's easier to see into the interior where the needle loss occurs. This is especially true if the shade source is new this year, because more needles than usual will be failing to produce enough food to be worth the cost to keep them.
All plants are equally affected (unless light conditions are different). If you have several plants of the same species, check to see if they all look the same. Not only will this pretty safely rule out any disease or insect problems (see our old post on how to tell if you're dealing with an abiotic or biotic plant problem), but in this case it means you have nothing to worry about if you have met the other two guidelines. As with the previous point, you can expect more needle loss in the shade (or at least more clearly visible needle loss), especially if the plant was not previously growing in the shade.
It is happening in the fall. This is a helpful hint, but not a surefire diagnosis. Serious problems can develop in the fall. But if your plant meets the first three criteria during autumn, that's another indicator that you have nothing to worry about.
If your tree or shrub doesn't meet these four criteria, it's time to do some diagnostic work to find out what else could be wrong. You can dig into this yourself with help from our blog series on plant problem diagnosis (to be brought from our old blog soon), or you can call a professional for their opinion. Better safe than sorry, so if you're identifying this problem for the first time don't hesitate to ask for help! Call us at 317.251.GROW to leave a question for one of our staff to get back to you.