As I look around the garden, it’s clear that fall is here. Most of the plants seem to be shutting down for the season. Leaves that were once bright green are now starting to turn brown. Plants that flowered well during the summer are hardly blossoming at all. And trees all around the neighborhood are starting show the colors that chlorophyll has been masking all season long.
I’d always thought that cool temperatures were the main cause of these changes but this year is confirming that this isn’t the case. We haven’t had many cool days, yet the plants are still shutting down. Right now the daytime temperatures are in the 80s but you’d never know it looking at the garden.
So what’s going on?
This year’s weather shows that while temperatures are part of the equation in causing plants to shut down, the primary factor is the length of day. If you think about it, it makes sense. Plants, especially perennials and trees, need to know when the seasons are changing so that they can prepare for winter. While temperature is one of the factors, it’s fickle, as this year shows. The one constant and sure way to know the seasonal change is by the length of day – or most specifically, the length of night.
There’s a whole biochemical process in plants that allows them to monitor the length of the period of darkness that they experience. And for plants that grow in temperate regions, once a certain length of night is reached, the plant begins to slow down and shut down. This dying off is called senescence. Leaves and other tissues periodically senesce all season long but in the fall, there’s a mass senescence triggered by the length of day – and to some degree, temperature.
I find it interesting that there are some plants that don’t seem to know that it’s time to senesce. My pots of ginger, mandevilla and tropical hibiscus aren’t growing as quickly as before but they still look fine. All of these plants are perennial but what sets them apart from the other perennials in the garden is that they’re tropical. In their native environment, they don’t need to prepare for winter so they haven’t evolved the same biochemical trigger as their temperate cohorts. I have to save them from the cold by bringing them inside.
This season of senescence shows how amazing plants are. They know what season it is and they know what they need to do to survive. As the days get shorter, the plants are shutting down, even if it is still in the 80s. The plants don’t care, but as someone who likes cool weather, here’s hoping that the temperatures start matching the day length!