Question:
What causes the trees to turn color in the fall? I always thought it was caused by cold weather: however, it’s been unseasonably warm, and trees are still turning color.Answer:
- Cold weather does not trigger the fall foliage colors in deciduous trees; rather, it is determined by day length, specifically the length of the nights.
- While temperature can influence the rate of color change, it is not the primary cause. In the Bay Area, the peak of fall color occurs from early November to early December. The main landscape trees that display color changes include Pistache, Japanese Maple, Crape Myrtle, Ginkgo, Raywood Ash, Flowering Pear, and Liquidambar.
- A few evergreen plants also change color, but this typically happens later, around late December or early January. For instance, Star Jasmine and Algerian Ivy turn maroon, while boxwood takes on a dull brown hue.
- The onset of the fall color season varies each year due to weather conditions. Although rare in our area, very cold nights can damage the vivid colors by rupturing leaf cells and causing them to die.
- Rain does not affect leaf color, but heavy cloud cover limits ultraviolet light. As a result, chlorophyll—the green pigment in plants—persists longer, keeping the leaves green for an extended period.
- Conversely, warm temperatures, can cause leaf cells to burn the stored sugars, leading to fewer red and purple pigments and allowing yellow and orange to dominate.
- The actual leaf color is determined by genetics. The green color gene is dominant, while red, yellow, orange, purple, and other recessive pigments become visible only when chlorophyll production decreases. Some trees have a single color pigment, whereas variegated trees exhibit several dominant pigments.
- Before the foliage changes color, a deciduous tree enters a dormant state, defined as a period in which a plant has ceased active growth.
- Among trees of the same species, the range of colors can vary depending on whether the tree was propagated from seed or vegetatively. Liquidambar, or American Sweet Gum, serves as an excellent example of this phenomenon—its foliage color remains uncertain until fall. Varieties such as Palo Alto, Burgundy, and Festival are grafted to ensure predictable foliage color.
- Plant stress can cause color changes in leaves that deviate from typical seasonal patterns, often due to drought or specific cultural practices. Examples of stress-related cultural practices include nutrient deficiency, irregular watering, and planting too deep. These unexpected shifts in leaf color during the growing season signal underlying issues. To achieve vibrant fall colors, ideal conditions include warm, sunny days, cool nights, and minimal wind or drought stress.
- This year has been exceptional for fall foliage.