Plants have evolved to sense and respond to changes in daylight as seasons shift, except those that grow at the equator, of course. Disrupting periods of darkness, such as by growing near streetlights that are on throughout the night, can affect a plant in a number of ways, but most are minimal if the plant is otherwise healthy.
Are Streetlights Bad for Plants?
The simple answer is yes. Deciduous green plants, particularly trees, measure light and detect when days are getting shorter and longer. This helps them decide when to go dormant in the fall and when to start to come out of dormancy in the spring.
The effect of streetlights on plants and trees can disrupt this important process. In the fall, take notice of trees under street lamps. The leaves right under the light tend to stay green longer than the rest of the tree. This delayed senescence is harmful because the tree is unable to take up the resources from those leaves before they die. Instead, they simply go straight from green and living to dead at the first real frost.
Streetlights can also be an issue for flowering plants. The length of the day for some flowering plants determines when they begin to produce buds and bloom. If you have certain flowering plants under a streetlight or security light, they may fail to bloom for this reason.
Planting under Streetlights
So, should you plant anything under a streetlight? Certainly, there are many cities and neighborhoods in which tree-lined streets co-exist with lights. Trees and streetlights are usually fine together if the tree’s other needs are well met, if they get adequate water and good soil with plenty of nutrients.
The harm a streetlight causes to a tree, keeping some leaves green too long, can cause small amounts of cumulative harm over time. But this is minimal and rarely an issue if the tree is healthy. The same can be said of shrubs. Keep your plants healthy, and place them out of the light if possible. You can also use special shields on lights, if they are your own private lights, that will allow them to illuminate an area without shining on plants.