Assessing sunlight

Fun facts about plant adaptations to sun and shade

• Silver foliage is common to many sun-loving alpine and Mediterranean plants such as artemesia (Artemesia spp.), snow-in-summer (Cerastium tomentosum) and Russian sage (Perovskia atriplicifolia). The silver colour reflects intense sunlight, keeping the leaf surface cool and reducing moisture loss.
• Hairy leaves. Look closely at lamb’s ear – it's soft because it has a hairy surface which is an adaptation that prevents excessive water loss due to transpiration as dry air blows across the surface of the leaves. In cool weather these soft hairs also insulate the leaves.
• Plants which thrive in low light conditions like hostas (Hosta spp.) and ligularia (Ligularia dentata) have adapted thinner and broader leaves to catch more sunlight. They also adapted the ability to use far-red light wavelengths (on the light spectrum) for photosynthesis.
• Thick fleshy leaves such as those common to cacti and succulents have an amazing capacity to store moisture in arid conditions. Their leaves also have a waxy cuticle (outer skin) which resists water loss.

FAQ

Q: How much light do I need to grow vegetables?

A: Any plant that produces a fruit (including vegetables) needs at least 6 hours of full sun per day but more is better for the best production. Open, south-facing sites that get sun all day long are ideal for growing vegetables and fruits. Vegetables that get less than 6 hours of full sun will be smaller and not produce as large a crop.

Cool season vegetables like lettuce, spinach, radishes, endive, arugula and kale can tolerate less light especially in the heat of summer.

Q: What about fruits?

A: Like vegetables, fruit-bearing plants such as apples, pears, plums, sour cherries, haskaps and beaked hazelnuts do best with full sunshine for most of the day. Currants, gooseberries, Saskatoons and raspberries will tolerate some light shade, but will not produce as much fruit.

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One of the most common problems with growing plants is not having the right amount of sunlight to meet the needs of the plants. The need for sun or shade varies a lot depending on the plant.

So, why is it that every plant has different needs? The reason is natural selection. In the plant world this means that plants with traits that can cope with their environment (including soil, temperature, moisture, nutrient and sunlight conditions) are more likely to survive and reproduce. They pass on these traits to the next generation and over time (often millions of years) these plants evolve to adapt and thrive in their native habitat.

To find out what your plant needs, start by checking the plant tag:

• Full sun – At least six hours of direct, full sun per day.
• Partial sun - Between four and six hours of sun per day.
• Partial shade – Between two to four hours of sun per day.
• Shade - Less than two hours of sun per day.

Now here is where it gets tricky. Even though plant tags tell you how much light a plant needs, this doesn’t tell the whole story. For example, plants that need full sun, like fruits and vegetables will be healthier and produce more when they get much more than the minimum six hours of sun a day. Other sun loving plants like Sedum, Salvia andVeronica will grow larger and produce more blooms with direct, long sunny days. On the other hand, plants like daylilies (Hemerocallis) and some irises do just fine with the minimum of six hours of sun a day.

While we all love a sunny garden, shade isn’t all bad. There are many beautiful plants that grow well in shady areas of your garden. But look closely: most shade-loving plants need some sun for photosynthesis, and dappled or filtered sun that reaches the ground through the tree canopy can provide that. Reflected light that bounces off nearby buildings and fences also helps to lighten shade. However, deep or dense shade found at the base of the north side of a fence or building, or under the canopy of a dense evergreen which receives no sunlight at all will not support most shade plants.

Direct sunshine in an east facing garden is the easiest on plants as mornings tend to be cooler in spring and summer. However, direct sun from the west can be harsh and hot especially in mid-summer and is not at all suited to some plants.

How to assess the light in your garden

Before you buy new plants or if you’re trying to figure out why your plants are not thriving, assess the amount of sun and shade your garden receives.

Note that light levels change over the years as trees and shrubs grow to their full height or when a new fence or building is erected. The loss of sun means that tomatoes growing in a partly shaded site will not produce as much fruit as ones growing in a garden in full sun. A former shade garden will soon die out after an old tree that shaded it is cut down.  It’s a good idea to assess your light levels every few years.

Light levels change over the course of the day too, as the sun moves to cast shade under trees, fences or buildings. So, if you have a garden that gets 4 hours of sun in the morning, followed by dappled shade for a few hours and then 3 more hours of sun in the afternoon, it still counts as 7 hours of full sun per day, enough to grow many plants that need sun.

In the prairie provinces and especially in northern Canada, the angle of the sun changes dramatically depending on the season. Need we remind you of the short, dark days of December and the glorious long days of July when the sun is high in the sky? A good way to assess the light available to your garden is by observing it. The best time of year to do that is during the growing season, preferably after the first frost free day in late spring. This varies from mid-May on the prairies to mid-June in the far north.

• Use a camera! Take a photo of your garden(s) every hour from dawn to dusk. Or, set your camera on a tripod and use the time lapse setting to take photos. Be sure to make note of the times you took photos.
• Or, simply observe the garden. Use a chart like this one to record your observations:

Key:

🌞 full sun

🌗 partial shade – shaded but bright; light may be reflected

🌒 filtered/dappled sun – overhead trees shade area, but some light breaks through

🌑 full shade – no direct sun

Now that you know your light conditions, there are things you can do to improve your situation. Start by going beyond the plant tag: consult a credible book or website to learn more about the sun needs for the plants you want to grow. When buying seeds or new plants, choose plants carefully: buy only the right plants for your site. To lighten shady areas consider pruning: remove a couple of lower limbs to raise the canopy of trees or prune out dense growth in larger shrubs. Transplant plants that are not thriving to a new site where light levels are more suited to their needs. It seems like a lot of work, but worth the effort for better flower, fruit and vegetable production and a healthy, sustainable garden.