How much light, and what kind of light, do plants need?
Different plants need different amounts of direct sunlight, which is when sunlight is streaming directly onto the plant. Full sun means the plant needs six or more hours of direct sunlight. Part sun is four to six hours of direct sunlight. Shade is less than four hours of direct sunlight.
Often there are ranges to describe a plant’s light needs, such as full sun to part sun, or part sun/shade to shade. Many plants in the latter range will not do well in afternoon sunlight, since it generally feels hotter than morning sunlight. Some shade plants may prefer dappled shade, where the light is filtered through tree leaves, rather than direct sunlight.
The type of habitat where a plant normally grows can give clues as to its light needs. A woodland is generally shade to part shade, while a prairie is full sun. A woodland edge might be sun to part sun.
Working with microclimates
When designing a garden, you need to take microclimates into consideration. Microclimates can make a normally sunny spot shady or make a sunny spot even hotter. For example, if you plant tall and short plants next to each other in a full sun location, the tall plants could shade the short plants depending on how they are positioned in relation to each other. This could cause the short plants to not receive the amount of light they need.
A brighter and hotter microclimate might be created by mulching with gravel or other stones or by planting next to a south or west facing wall. These types of hardscape can influence which plants will do well and which will wither. However, you can use microclimates to your advantage and put in plants that normally would not thrive in a particular location but can because of the extra shade or brighter conditions.
If you want to grow woodland plants, but the garden is in full sun, you might be able to find a microclimate of shade, at least in part of the garden. This might came from shade cast by a lone tree, the side of a house, or a dense shrub line. You can also plant a shrub or use the placement of a container or larger garden ornament to create a small area of shade.
If you don’t have any shade, but you want early spring blooms, there are some spring plants that can grow in full sun, like shooting star, pussy toes, Jacob’s ladder, and blue-eyed grass.
Seasonal changes
Because of the north/south shift of sunlight over the year, the number of hours of light plants receive over a year can change. This can make it challenging to know, for example, if a spot is full sun or part sun. In front of my house, the north-facing foundation beds shift from part sun to shade in the winter to full to part sun in the summer. In early spring, I planted a part sun to shade plant there thinking it would only receive morning sun. By midsummer, the plant was getting morning and afternoon sun, and I had to move it because its leaves were getting burned.
When planning a garden, put the strongest consideration on the amount of light available from June to August. Look at the site several times a day to get an idea of when direct sunlight is reaching the space.
If you need to plan a garden before you can analyze the midsummer light, keep in mind that the trees will leaf out and the sun will shift to be overhead rather than toward the south. This means shade from north-facing walls extends out farther in the winter than in the summer.
Influence of soil moisture
A plant’s tolerance to light might depend on how wet or dry the soil is. Some plants that prefer shade to part sun can tolerate full sun as long as the soil doesn’t dry out.
Light requirements of specific plants
To learn more about needs of specific plants, there are several online sources you can turn to. Here are some reliable ones:
- Prairie Nursery
- Prairie Moon Nursery
- New Moon Nursery
- Missouri Botanical Garden
- Plants.USDA.gov
You might find information that differs among the sites, but this can be helpful as you plan a garden. If all the websites agree, you’ll know you won’t be taking any chances with a client’s garden. If the garden is for yourself, you might be willing to take a chance with a plant that only one of the sites lists as full sun or full shade to see if it will grow in your garden.