This is a common worry, so if you’ve worried about it, you’re not alone.
Mountain Hemlock as an example, though what we’ll talk about can be applied to many conifers. The newer needles are pale, which indicates weakness. The inner needles are starting to yellow. Is this a problem?
Not necessarily.
Weaker trees do lose more needles. Just as the new growth is hardening off we tend to see a bunch of them yellow and fall off. This is just replacement and not a cause for worry. Weaker trees tend to do this earlier in the year, however.
Same tree. A couple small shoots are yellowing in the center of the photo, and are dying. Is this a problem?
Also, not necessarily a problem, at least in terms of disease or pest.
Old, established trees have a lot of shoot density. If, for whatever reason, the tree is weaker, that old tree will self-select particularly weak shoots and cut them off from fluids, killing them. Improvement in horticulture for the tree—any number of things from routine maintenance reinstated, low fertilizer increased, watering adjusted, a repot next spring, or simply moving it to a spot in the yard better suited to the species—may well correct the problem next year.
So even though this tree is weak, and the color could be better, the shoot length is OK and with some adjustments should make a fine recovery.
Here’s a stronger Mountain Hemlock. Note the vigor of the tip growth and the more intense glaucous blue-green foliage. But also note the interior needles dying off.
There’s nothing wrong here. This is normal, healthy needle replacement.
Same tree. Again lots of yellowing needles on the interior. But lots of new foliage to replace what it’s losing. Strong trees will lose less old foliage. Also, the more damaged old foliage is from pest or disease, the more likely it is to be shed.
One thing you might have noticed after repotting a weak or recently collected conifer: a month later, half the needles yellow and fall off. This might make you panic, but it’s actually the best sign that you have root growth. If the newest foliage remains green and good-looking, you’re winning.