Hi, 

Could you provide a photo or the back of the yellowing leaves? We suspect that it could be a scale insect problem. Other than that the trees are probably stressed from heat. Canaan Fir really do better in a cooler less humid environments and typically grow in the mountains of West Virginia for their southern range or in the northern parts of the eastern US. Especially with them being in pots, they are probably extremely heat stressed. Its hottest zone is around 6a so the pot it is mimicking a warmer environment on top of probably being a bit too warm for it in MD.

Stressed plants are more prone to disease and pest problems, so even if we can't pinpoint the needle problem to a scale insect, the trees seem stressed regardless. 

Have you been supplementing water and fertilizer? With the pots the soil will be drying out much faster than in the ground and the nutrients will be leaching out as you continue to water. 

If you can send additional photos and maybe a zoomed out photo or the entire tree/pot that would be helpful as well. 

Most likely though, we may need to recommend a better evergreen for the planters. 

Emily

Thank you for the photos. Although hard to see clearly, it does  look like scale insects are present. Specifically, based on their appearance in combination with host plant preferences including fir, one of the armored scale types: likely Cryptomeria Scale or perhaps Elongate Hemlock Scale (both species are on that same linked page).

Scale crawlers, which are freshly-hatched juveniles, are the most vulnerable life stage when it comes to using any type of contact-kill insecticide (in comparison with other pesticide options like growth regulators and systemics). The window of time for second-generation crawler emergence overlaps for both species, so in that regard, knowing which of the two it is isn't critically important, so a horticultural oil application might work. Oil would, however, risk removing some wax from the leaves, making them greener and losing any blue-gray tones. You would also need to be careful about applying any pesticide spray, especially oil, during heat above 85 degrees because it could "burn" foliage (called phytotoxicity). With scale, since they are generally hard to suppress, systemic products are probably more effective overall instead of contact ingredients since their timing of use will be less critical to their efficacy.

Due to a Maryland pollinator protection law, it would not be legal to use a systemic insecticide on outdoor plants which contains active ingredients in the neonicotinoid class. ("Neonic" active ingredients, like imidacloprid and dinotefuran, are probably the only types, or at least the most widely-used types, available for scale insect control.) They can be applied by a certified pesticide applicator if you wanted to hire one for this purpose. Since that's probably not practical in this situation, though, you may need to rely on repeat horticultural oil treatments to kill crawlers and to hopefully suppress overwintering scale as well; always follow product label directions for usage. Successful scale management might take over a year to achieve.

Dark-colored pots can certainly heat-up more than light-colored pots, but that's not the only factor, since the root ball is entirely above-ground, it has little insulation overall from fluctuating temperatures, both summer and winter. While firs are generally quite cold-tolerant, they still would normally have roots well-insulated from rapid change (both in warmth/chill and drying out) when growing in the ground. Sinking pots into the soil will probably be much more effective than changing pot color alone if heat is one of the stressors impacting their health.

They may very well dry out daily depending on weather, root mass size, and potting mix water-holding capacity, but make sure the soil is becoming somewhat dry to the touch at least a couple inches deep into the container (the depth to check depends a bit on container size) before watering again. This lets the soil "breathe" a bit so roots have access to oxygen and aren't always growing in too-damp conditions.

How often a potted plant would need fertilization depends on growth rate, time of year, and how nutrient-retentive the potting mix is. Water-soluble nutrients like nitrogen will leach-out of the mix faster with frequent irrigation or heavy rains than other nutrients. Pelletized slow-release fertilizer formulations last much longer, as they are designed to do, than liquid-applied fertilizers. Now that we know scale insects seem to be a problem, we would not recommend fertilizing until the scale are reduced, since pest insects often benefit from plants receiving supplemental nutrients. Plus, as we enter late summer, winter-hardy plants are preparing their tissues internally for winter and are not producing much, if any, new above-ground growth, so their nutrient demands will decline. Fertilization could wait until spring in that case. There is no particular formulation recommended for fir specifically; any well-balanced formulation should be fine, and of the three major nutrients (N-P-K), nitrogen is the one used in the greatest quantity by plants (especially for those not grown for flowers), so you can select a formulation that has N as the highest part of the N-P-K ratio.

Miri