Temperatures are feeling more like fall, and the trees are following suit. According to the Iowa DNR Fall Color Report, much of the state experiences peak color in mid- to late October.
Mark Vitosh, a forester at the Iowa DNR, joined Garden Variety to share how weather conditions impact when and how fall colors appear.
“Depending on what colors you want to see, especially if you want to see vibrant purples and reds, we need cool evenings and clear days," Vitosh said.
Moisture is also an important factor in how vibrant the colors will appear. Dry conditions can cause leaves to look more bleached, or cause browning.
Looking more specifically at the science, the phytochromes – receptors inside the leaves – can sense that it’s time for the tree to change when the days start getting shorter and the nights start getting longer.
“The thing you got to remember is these trees that are changing color are deciduous trees,” said Vitsoh. “So we live in a temperate area, and we know we have specific seasons, and those plants can't survive winter if they have leaves on them — if those leaves are actively growing, they're not going to survive.”
To survive the off-season, the tree has to go dormant in the colder months. As part of that process, the plant starts to produce a corky substance that plugs up the vascular system, leading to less carbohydrates and water in the leaf. This leads to less production of chlorophyll, the compound that creates the green pigment.
“The neat thing is, there's other pigments that have been there all year — called carotenoids — they've been there all year, just waiting their turn. And that's our yellows and our oranges.”
Depending on the species, other colors emerge.
“Some sugars get trapped in those leaves. On certain species that turn purple and red, when those sugars are trapped during the day, at night that actually produces another type of pigment called anthocyanin, which gives us our reds and our purples,” Vitosh said.
In years with consistently warmer temperatures, the process for going dormant may slow down, so the transition to fall colors doesn’t happen as quickly.
“You're always going to have that trigger of light, or lack of light and darkness, that is the start,” Vitosh said. “But then weather, temperatures, rain, freeze, all that can have a big impact on how long things last.”
For more horticulture advice, listen to Garden Variety, hosted by Charity Nebbe.
