Growing Turf in Low Light Dr. Aaron Patton provides insight

Growing Turf in Low Light

A Presentation by Dr. Aaron Patton

by Meta L. Levin

Turf needs light to grow, but conditions often mean there is too little or poor quality light available. Sometimes, property owners have unrealistic expectations of where it is and is not possible to grow grass on their lawns. If you are nodding in understanding, then you would have been right with Aaron Patton during his Turf Education Day presentation, “Growing Turf in Low Light, Shaded Environments.”

Patton, interim department head and professor in Purdue University’s Department of Horticulture and Landscape Architecture, took attendees through a basic primer on photosynthesis, the process by which plants trap the sun’s energy to build carbohydrates to use as stored energy. It is a lightdependent action and that light comes from the sun.

At this point, we will need a glossary, albeit a short one: 1. Shade environment: an area with reduced light, moderated temperatures, low air movement, high humidity, and longer periods of leaf wetness

2. Photosynthetically Active Radiation (PAR): The term for wavelengths of the visible light spectrum, between 400 and 700, which plants use for photosynthesis. 3.

4. Photosynthetic Photon Flux Density (PPFD): The units used to measure PAR

Daily Light Integral (DLI): A 24-hour measurement of PAR

So, how much daylight do grasses need to grow? “This is a question we are still trying to answer,” says Patton. It is dependent on several factors, not just sunlight, including temperature, grass species, mowing height, and plant nutrition.

In fact, the shorter you mow the grass, the more light it will need, says Patton.

There are apps, such as Sun Seeker, that help track the amount of sun on different spots. In the case of Sun Seeker allows you to track the sunrise and sunset times, as well as check the sun position and solar path over the lawn. The cost is $9.99, if you are interested.

An app like this will help you not only show the path over the lawn, but give you an idea of where the most shade is coming from, where it’s going to be in a couple of months, and where it was a few months ago.

“Most grasses prefer full sun, except on the scorching

Measuring photosynthetically active radiation (PAR)

days of summer,” says Patton. “It depends on which turf species you grow.” The rest is not, he says, down to a science “as much as we would like.” Tools like the apps do help turf specialists go in the right direction.

It is frequently the case, says Patton, that a resident’s front yard looks beautiful, but not so the shaded back yard. If a landscape contractor tells the client there is not enough light in the back yard to grow grass turf, that is much different than pulling out a meter that can measure the PAR or DLI, which “shows professionalism and gives your argument more credibility,” says Patton.

Not all grass species have equal shade tolerance. Even so, it takes at least four hours of sunlight a day to grow the most shade tolerant turfgrasses. “Keep in mind that six hours of sunlight between 6 am and noon is much less than six hours of sunlight between 9 am and 3 pm, as light intensity is greatest at midday,” says Patton. For instance, Bermudagrass needs eight or more hours of full sunlight daily and Kentucky bluegrass, perennial ryegrass, and zoysiagrass all need six or more hours of full sunlight daily. (continued on page 20)

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Turf Update —

(continued from page 19)

While some sunlight can filter through tree leaves, its quality is less than full sunlight. More far red light passing through the trees to the turf, means that the turf will not grow as well under the shade tree. Plants absorb red light and reflect or transmit most far red light, which is not useful in photosynthesis. “A typical symptom of too much far red light is leaf elongation,” says Patton.

Chlorophyll absorbs the red and blue wave lengths. We see the tree and turf leaves as green, because that is what is reflected at us. “If the turf gets too much far red wave lengths, it will cause turf to be thin and weak,” says Patton.

Believe it or not, there are different types of shade.

When light is filtered through trees, it is called dappled shade. It creates a moving pattern of sunlight and shade, decreasing both quality and quantity of sunlight on the turf.

Open shade, on the other hand, typically is found in a north-facing yard and is cast by an adjoining wall, fence, or building. Known as neutral shade, the distance of the shadow varies with the season and affects the light quantity, but not quality.

When open shade is further obscured by trees, it is known as medium or dense shade and decreases both the quality and the quantity of the light that can reach the turf.

Any of these types of shade can be dry shade, which occurs because most trees have shallow roots, removing nutrients and water from nearby turf.

Any types of shade also can be wet shade if the occur in poorly drained or low spots. This is,

however, less common under mature trees and more common adjacent to buildings.

Beset by lack of quality light and/or poor-quality light, turfgrass may eventually die in shade. Often, the lighting situation is exacerbated by disease, tree root competition for moisture, and foot traffic.

So, what do you do?

Start out by planting trees with turf in mind. For instance, with heavy shade trees, like maple, elm, linden and tulip, you can expect less than 15 percent light transmission. Moderate shade tree species, such as oak, poplar, and hickory, provide slightly more light transmission. “A small improvement in light penetration will help long-term turf maintenance,” says Patton.

Use technology to help estimate the amount of shade. There are, like so many other things, apps for that.

If a tree is causing the low- and poor-quality light situation, Patton recommends trimming lower branches and removing understory plants to improve air movement. If the property owner agrees and the proper permit is granted, it may be necessary to remove some trees altogether to improve the turf.

Once the situation is established, follow some management guidelines. Shaded areas, says Patton, need less nitrogen fertilization, so he recommends reducing the nitrogen fertilization by 50 percent in shaded areas. These areas also need mowing less often, because they grow less and don’t tolerate traffic as well.

These shaded grasses have a thinner leaf cuticle and are prone to injury from traffic. They also are slower to recover from damage.

When irrigating a shade area, remember that turfgrass under trees may need more irrigation in summer and early

fall months, but will need it less often next to buildings.

Patton recommends seeding shade tolerant grasses in shaded areas to increase turf cover. Use such grasses as fine fescues (Chewings, strong creeping red, or slender creeping red fescue) and tall fescue for most shaded areas, and rough bluegrass or supina bluegrass, in wet areas (warning, neither of these bluegrasses is well adapted to summer).

Some common weeds are more problematic in shade. They include: annual bluegrass, common chickweed, ground ivy, nimblewill, rough bluegrass, and wild violet. On the other hand, weeds, such as crabgrass, goosegrass, and prostrate spurge are less challenging.

When using an herbicide, Patton warns to be sure to check the label for any restrictions under the drip line of trees or in areas with exposed tree roots.

Typically, shaded areas have poor air movement, higher humidity, increased leaf wetness, and plants with thinner leaf cuticles, making them more susceptible to diseases, such as powdery mildew, leaf spot, melting out, and microdochium patch. Moss and algae can be secondary problems caused by thin turf.

Plant growth regulators (PGRs) can improve the quality of turf growing in shaded areas for a variety of reasons:

They reduce the growth rate, so there is less leaf elongation.

They increase carbohydrate reserve, thus giving the grass more energy.

They increase density, so there is more photosynthesis.

Of those PGRs that can help, Patton suggests trinexepac-ethyl as the “best option”. The PGRs aren’t readily available to homeowners, but can be applied by lawn care professionals.

Using artificial light in areas of high economic importance, like a professional stadium, is expensive and provides only a small benefit, he says.

In lieu of grasses, Patton notes that mulching around tree bases and using shade tolerant ground covers around trees are useful alternatives in heavy shade areas.

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