UNITED STATES—It might seem like autumn color is a bit early this year. In the wild, where there is no irrigation, native box elders, and maybe some of the cottonwoods and sycamores, are already yellowing. The box elders are already dropping some of their foliage. However, this is not caused by an early chill. It is caused by late warmth after an otherwise mild summer, and might inhibit better color later.
It is still impossible to say for certain. There are so many variables that affect autumn color, such as temperature, humidity and day length. Some of the plants that develop color late, like flowering pear trees in irrigated landscapes, might not be bothered too much by the same variables that are troubling wild box elders now. Where autumn weather is so mild, color is unpredictable anyway.
Sudden cool weather after mild weather late in summer would have been better for autumn color. (Warmth through the middle of summer is no problem.) Foliage lingers best if the weather stays cool without getting too cold. Wind and rain will eventually dislodge foliage. Sheltered sweetgums might hold their foliage until just before it gets replaced by new foliage late in the following winter.
The earliest trees to color, as well as those that are notoriously less reliable for color, are the most likely to be inhibited by the weather this year. Besides box elders, cottonwoods and sycamores, other marginally colorful trees like willows, walnuts, maples, elms, redbuds, smoke trees and deciduous oaks, may develop bland color. Mulberries and tulip trees, though early, are more reliable.
Sweetgums (which are also known as liquidambars) , flowering pears, Chinese pistaches and maidenhair trees (which are also known as gingkos) are the most reliable trees for color locally. Maidenhair trees turn remarkably bright yellow. The others can be various shades of bright yellow, orange, red or burgundy red, (although Chinese pistaches do not not often get burgundy color here). Crape myrtles have the potential for the same color range if conditions are right.
Highlight: flowering pear
Now that they have been so popular for so long, the deficiencies of flowering pear, Pyrus calleryana, (which is also known as Callery pear), are becoming increasingly evident. The dense branch structure and compact form that are so appealing while trees are young compromise structural integrity as trees mature. Symmetrical trees can be severely disfigured if limbs get broken by wind.
In the past, flowering pear had been quite resistant to disease and insect infestation. However, now that there are so many of them within minimal proximity of each other, it is nearly impossible to avoid infection with fire blight. To make matters worse, they share this very contagious bacterial disease with fruiting pear, evergreen pear (fruitless) and apple (fruiting and flowering crabapple).
Otherwise, healthy and well structured trees provide some of the best autumn color for mild climates. Foliage that was rich green through summer becomes bright yellow and orange on a weak season, with more bright red and burgundy with a good chill. The white spring bloom is likewise more colorfully profuse after a colder winter, although some find the fragrance to be objectionable.