Question:
What causes trees to turn color in the fall? Is it the rain or the cold weather? I have a friendly wager with my neighbor that it's the cold weather.Answer:
- It's neither the rain nor the cold that causes the autumn foliage color in deciduous trees. Instead, the trigger is the day length or, more correctly, the length of the nights.
- Temperature does play a role as it accelerates or slows down the rate of change. In the Bay Area, fall color peaks in early November through early December. Pistache, Japanese Maple, Crape Myrtle, Oaks, Raywood Ash, Flowering Pear, and Liquidambar are the primary landscape trees that change color.
- We also find a color change in a few evergreen plants. Star Jasmine and Algerian Ivy turn maroon while boxwood is dirty brown.
- The beginning of the fall color season shifts yearly due to the weather. Although it's unlikely to occur in our area, very cold nights can ruin the vivid colors by causing cells in the leaves to rupture and die.
- Rain is never a factor in leaf color, but heavy cloud cover limits the ultraviolet light, so Chlorophyll, the green pigment, lasts longer; hence, leaves stay green longer.
- Warmer temperatures cause leaf cells to burn the accumulated sugars, so fewer red and purple pigments are formed, with yellow and orange being the predominant colors.
- The actual leaf color is determined by genetics. The dominant color gene is green, while the red, yellow, orange, purple, or other recessive pigments are only apparent when Chlorophyll isn't produced.
- Many trees have only one color pigment, while trees with variegated foliage have several dominant stains.
- A deciduous tree goes dormant before the change in the foliage color. Dormancy is defined as a plant that stops activity from growing. Among the same species, the range of colors can and will vary.
- It depends on whether or not the tree was propagated from seed or a vegetative part.
- Liquidambar, an American Sweet Gum, is an excellent example. The foliage color is unknown until the fall. Palo Alto, Burgundy, and Festival are grafted varieties sold for their predictable foliage color.
- Unrelated to weather, plant stress can cause the foliage to change color out of season or very early. Drought is the usual cause, but other cultural practices can be an issue. This type of change in foliage color is an indicator of a problem.
- The ideal conditions for fall color are warm, sunny days and cool nights with little wind or drought stress. This has been a fabulous year for the fall color.
Question:
Several branches of my apple tree have just blossomed again. Does this happen often, as it's a first for my trees? What is going to happen now?Answer:
- With our varied microclimates, this occurs more often than you might think when the right conditions arise.
- The combination of warm days and cool nights has caused your apple tree to think it has gone through a mild winter and spring has arrived.
- There is little chance that apples will form. The chief pollinator for apples is the honeybee, and they're inactive at this time.
- This action poses no danger to the tree, nor will it alter next year's flowering cycle.