We all know that deciduous trees—oaks, maples, and the like—lose their leaves in the fall. But what about conifers? They’re supposed to be evergreen! Should we be worried if we see lots of brown needles on our pines and firs?

As is frequently the case with questions about gardening, the answer is “it depends.”

It’s completely normal for pines, in particular, to lose some needles in the fall. Most pine needles live for about five years. Then they die and fall off. That’s why pines have lots of green on the outside of their branches, but just bare twigs and branches in their interiors. If your tree looks like this, there’s no reason for concern:

Some conifers are deciduous. Yes, we call conifers “evergreens” but the terms are not as interchangeable as we’d like to think. Some broadleafed trees are evergreens, such as magnolias, olives, and citrus; they just don’t survive our cold winters. And some needled trees, conifers, lose their leaves every fall. Larches (Larix sp.) and Dawn Redwoods (Metasequoia glyptostroboides) might be grown in Colorado, and several species of cypress are also deciduous.

However…

If your pine tree’s branches are turning brown from the tip inward, or if one side of the tree is uniformly turning brown, you have a problem:

The most likely cause of needle death is root issues, and the most likely cause of root issues is incorrect watering. Too much water will rot the roots and drown the tree. Too little water will kill the roots. Either way, the tree will be unable to replace water that evaporates from the needles, and they’ll turn brown and die.

Planting too deeply, abrupt discontinuities in the soil (as from amending the planting hole but not the surrounding soil), failure to remove the wire and burlap when planting, and soil compaction are other possibilities.

Sometimes, the needles on one side (usually the southern or western side) of a conifer turn brown. This is winter injury, caused by sun drying out the leaves while the ground is frozen, limiting water uptake. Watering well before the ground freezes, and on warm days in midwinter, helps alleviate this problem

Herbicides can also kill needles. Spraying weed killers on a windy day may lead to unintended consequences as the cloud of deadly mist drifts across your yard—or over your fence. (Even applying a broad-leaf weed killer to your lawn can kill shrubs and trees growing in the grass—the chemical doesn’t distinguish between dandelions and desirable plants.)

String trimmers are easy ways to mow down tall weeds or grass, but keep them away from tree trunks. The damage they do can eventually gird the trunk and kill the tree.

Biological factors also come into play. A number of foliar diseases attack conifers. Needle cast diseases are caused by a several different species of fungi. Spores are spread by wind. Damage may not appear for several years, as the fungus completes its life cycle. Eventually, infected needles turn red brown to a straw-yellow, and fall off.

Given how common pines are, it’s not surprising that a lot of insects chew on pine needles: tiger moth larvae, sawflies, midges, budworms, and more. Scale insects and Giant Conifer Aphids suck sugary sap out of the needles, which then die and drop off. The aphids are only a quarter inch long, but that really is huge for an aphid. (Here you can see them being “shepherded” by ants.) Mites (tiny creatures related to spiders) also suck juices, and can be identified by the fine, delicate webbing they leave behind.

Most of these insects do little damage, as they’re rarely present in large numbers. However, if the tree is already stressed from other causes, populations may soar, resulting in defoliation or even death. Then there are the pine beetles, which have killed extensive tracts of trees throughout the west.

The best way to prevent pests or diseases from killing your conifer is to take good care of it. Research your trees and meet their needs. Not all conifers have the same growing requirements. For example, a blue spruce prefers damp, but well-drained soil, while giving that much water to a piñon pine will kill it. Invest a bit of TLC and you’ll be enjoying the winter green of conifers for years to come.