Shade
What are the possible problems?
 
Characteristics of shade plants

1) lower PS rates under full sun than non-shade plants
2) light-saturated at low-light intensities
3) PS is higher under very low light conditions than other non-shade plants.
4) respiration is very low.
5) shade leaves larger in area and thinner than sun leaves
6) reduced carbohydrate levels
7) lower C/N ratio than full sun plants

On a dry wt. basis shade leaves have more chlorophyll and less total proteins, RUBP enzyme than sun leaves

High light intensity breaks down chlorophyll. So there is constant synthesis of chlorophyll going on.

 

Shade/Developmental responses of turfgrasses


1) - thinner leaves with less weight/area
2) - reduced leaf width
3) - thinner leaf length
4) - thinner shoot density
5) - less tillering
6) - more upright growth
7) - reduced rooting
8) - increased root/shoot ratio

 

Shade microenvironment

1) - reduction in light intensity and quality
2) - moderation of temperatures both daily and seasonally
3) - reduced wind movement
4) - increased RH why (lack of air movement)
5) - increased CO2 level (why?)
         1. reduced wind movement
         2. reduced P n
6) - competition by trees for water and nutrients not to mention light
7) - increase soil moisture (less ET - less cooling needed
                                                     - less removal of H2O vapor by wind)

 

Why do temps moderate during the day? Why at night? Ave air temp sun -74, shade - 62; max air temp sun - 96, shade- 73; min. air temp sun - 52, shade - 53.

 
Shade Environment
 
Estimated that 20 to 25% of all existing turfs must be maintained under some degree of shade from buildings, shrubs, or trees.
 
Remember, only a small portion of solar energy reaching earth is used in the process of photosynthesis, 1 to 5% on a yearly basis and 3 to 10% during maximum active growth.
 
For cool-season grasses maximum irradiance for optimum photosynthesis is between the 116 to 233 W m-2. 339 to 681 µmol m-2 s-2
 
For warm-season turf grasses maximum irradiance for optimum photosynthesis is between the 390 to 465 W m-2.
 
Remember, photosynthesis in C4 plants can function under lower concentrations of CO2 in the atmosphere than can C3 plants using the Calvin pathway.
 
The optimum temperatures for Calvin cycle photosynthesisare 10 to 20° C.
The optimum temperatures for the C4 pathway are 30 to 40° C.
 
Under high temperatures and high photosynthetic irradiance, dry matter production for C4 plants can reach 30 to 50 g m-2 day-1.
 
For cool season grasses maximum dry matter production is generally 2 g m-2 day-1.
 
The photosynthetic-respiratory balance is a critical factor in shade adaptation.
Net photosynthesis must exceed respiration if the plant is to survive.
 
 
COMPONENTS OF SHADE
 
Light Quantity
 
Solar radiation (SR) or shortwave radiation refers to radiation originating from the sun with wavelength between 300 to 3,000 nm.
 
The solar spectrum can be divided into three main regions: ultraviolet (300-400 nm), visible (400-700 nm), and the infrared (700-3000 nm).
 

Beard measured light intensity (cal cm-2 min-1) at noon in full sun (1.6 cal cm-2 min-1) and on shade at 0.15 cal cm-2 min-1.

Only 37% of the energy in sunlight is within the wavelengths used useful for photosynthesis; 62% is infrared (>700 nm), and the remaining remaining 0.6% is ultraviolet (200 to 400 nm).

 
Sunlight reaching the ground at sea level has a PAR intensity of about 1,800 to 2,300 µmol per meter squared per second on a cloudless day. Saturation point for photosynthesis ranges 534 to 1,072 µmol per meter squared per second for cool-season grasses and from 1,794 to 2,139 µmol per meter squared per second for warm-season grasses.
 
SR varies from 0 at night to over 900 and 1200 mW cm-2 at solar noon on clear days in temperate and tropical regions, respectively.
 

Light intensity has a profound effect on plant growth and development.

The PAR available for plant growth varies from month to month, day to day, and from minutes to minute depending on the time of year, angle of sun, day length, and cloud cover.

 
The percentage of incident PAR that is intercepted and absorbed by the turf canopy varies with the kind and structure of the canopy.
 
Shorter cut turfs in shade would receive a higher percentage of PAR, but this is in opposition to the turfs physiological response to shade.
 
Light Quality
 
Tree canopies alter the spectral composition of available light and reduce the total to radiance on underlying turf for photosynthesis.
 
Light quality can be altered to the point that adequate photosynthesis cannot take place.
 
Light from the shade of buildings of neutral color generally are higher in the blue lightwaves.
 
Spruce, Oak and Maple trees absorb large amounts of blue ligh leaving what is often called "red" Shade.
 
Spectral distribution of light differs in relation to the distance from the tree trunk.
 
As the canopy becomes denser, all wavelengths are inhibited to a greater degree, particularly those in the blue region.
 
Warm-season turf grow better under shorter wavelengths (blue) making them more susceptible to poor quality light.
 
It may be possible to grow acceptable quality bermudagrass and Kentucky bluegrass in no less than 40 to 50% of full sunlight if adequate blue light is available. This was confirmed in an independent study growing turf under blue transmitting panels.
 
Microenvironment
 
While light quantity and quality are very important, the most limiting factor in turf adaptation to shade is disease susceptibility.
 
Conditions of high humidity, still air and high soil moisture often are more favorable to disease organisms than the turf
 
Generally turf managers should be concerned with the following factors in shade.
1. Reduced photosynthetic irradiance
2. Increased disease development
3. Increased susceptibility to turf injury from wear an environmental stress
 
Shade adapted weed species can also be very competitive with turf in shaded areas.
 
Shade as a stress factor
 
 
Overall plant response of turfgrasses when grown under low light intensities are as follows:
1. Higher chlorophyll content
2. Lower respiration rate
3. Lower PS compensation point
4. Lower carbohydrate reserve
5. Lower C/N ratio
6. Reduced transpiration rate
7. Higher tissue moisture content
8. Lower osmotic pressure
 
A common observation in shading studies is that shaded plants become elongated, weight per-unit area leaf area decreases, leaf area per unit shoot increases.
 
Another observation in shading studies is that turf reduces root and rhizome growth proportionally more than shoot growth.
 
Shading may severely reduce the number of leaves, tillers and rhizomes produced by the plant.
 
Moderate shading may increase tillering.
 
In a study of sun and shade dicot species, light compensation and saturation points for sun species were 9 to 13 W m-2 and 168 to 210 W m-2, respectively.
 
Shade species reach their light saturation point at 84 W m-2 with some species showing a maximum saturation as low as 34 to 42 W m-2.
 
Morphological changes
 
leaf structure
 
Full sun intensity favors development of leaves with several layers of long palisades cells.
 
Under shading, leaves have more spongy parenchyma tissue.
 
Leaves in sun also have a larger number of stomata, thicker cell walls and cuticle, and fewer and larger chloroplasts.
 
Generally high solar radiation decreases plant height but increases stem diameter and dry weight.
 
Plants grown in shade are taller with thinner stems and lower dry weights then plants exposed to full sunlight.
 
In a classic study of Wilkinson and Beard (1974), red fescue grew more horizontal under low photosynthetic irradiance while Kentucky bluegrass exhibited a more vertical growth habit.
 
Clipping weights of both species decreased & moisture increased under shade.
 
Chlorophyll concentration per dm-2 decreased while chlorophyll concentration per gram increased as photosynthetic irradiance decreased.
 
Red fescue produced greater shoot weight than Kentucky bluegrass under low light.
 
Kentucky bluegrass produce less leaf area, fewer shoots per cm2, and fewer tillers per plant as the light intensity was lowered, while red fescue produced equal numbers through each step.
 
Pennlawn red fescue was superior to Merion Kentucky bluegrass in low photosynthetic irradiance only in terms of shoot growth below the cutting height or production of verdure.
 
McBee and Holt (1966) found shaded grasses that grew poorly during summer months improved in density and groundcover in the fall.
 
They suggested that shading would be less detrimental if accompanied by a reduction in temperature.
 
Anatomical changes
 
In the Wilkinson and Beard, 1975 study Merion Kentucky bluegrass displayed a decrease in cuticle thickness and vascular and support tissues under reduced photosynthesis irradiance, while Pennlawn red fescue did not.
 
Stomatal density of both species decreased under reduced light, but the stomatal pore length did not very with photosynthetic irradiance.
 
Merion Kentucky bluegrass had increased thylakiod and grana stack development within individual chloroplasts at reduced light.
 
Chloroplast ultrastructure remained unchanged in Pennlawn red fescue.
 
Nutritional changes
 
In a study by Burton et al. (1959), they found shading reduce the growth and yields of coastal bermudagrass particularly when heavily fertilized.
 
Grass receiving 1800 kg of N ha-1 yr-1 showed a much greater reduction in growth and yield then that given 224 kg of N.
 
Schmidt and Blazer (1967) found liberal nitrogen fertilization of 'Cohansey'psd bentgrass decreased carbohydrates and inhibited root development when grown in the shade.
 
When N nutrition is low, shade can stimulate shoot dry matter yields and increase N concentration of the shoot.
 
However, when N is abundant increasing shade results in almost a linear decrease in shoot dry matter.
 
Shade Ecology
 
Plant competition
 
Turfgrasses grown in high populations under low light have reduced leaf width and are generally smaller. This is one of the reasons for a high seeding rates for coarser texture species such as tall fescue.
 
Tree roots
 
Tree roots can reduce the growth and vigor of most turfgrasses even when water and nutrients are maintained at optimal levels.
 
Kentucky bluegrass is more sensitive to tree root effects then red fescue, rough bluegrass and perennial ryegrass.
 
Red fescue is strongly competitive, growing equally well with or without tree root competition.
 
Allelopathy
 
Allelopathy is the chemical inhibition of neighboring plants.
 
There have been a number of reports of allelopathic substances being produced by Barberry, horse chestnut, rose, lilac, viburnum, and mockorange.
 
There are probably many more reported observations of allelopathic substances from grasses that impact the early growth of various different dicot species or other grasses.
 
Turfgrass quality
 
Shade adaptation
 
Shade adaptation of a turfgrass ground cover is influenced by a complex of microclimatical, pathological, and physiological responses.
 
1. Reduced the irradiance
2. Tree root competition for nutrients and water
3. Microclimate that favors disease activity
4. Succulent grass tissue
5. Reductions in shoot density, root growth and carbohydrates
 
Plants capable of shade adaptation develop a higher photochemical efficiency. This generally means they have a lower light compensation value and begin to use light at lower levels than non-adapted species.
 
The photosynthetic-respiratory balance is a critical factor in shade adaptation.
 
Wilkinson et al. (1975) found that Merion Kentucky bluegrass and Pennlawn red fescue responded similarly to reduce light intensities in terms of net photosynthesis, light saturation levels, and light compensation point, but these factors could not be associate with the ability of Pennlawn red fescue to provide a more desirable turf than Merion Kentucky bluegrass in the shade.
 
Dark respiration of individual plants of Pennlawn was reduced at the lowest light intensity, but dark respiration of Merion was not reduced.
 
Karnock and Augustin (1981) demonstrated that Glade Kentucky bluegrass (shade tolerant) exhibited a high rate of photosynthesis under reduced light than Merion Kentucky bluegrass (shade in tolerant).
 
Disease problems
 
Higher relative humidity, extended periods of dew cover, and reduce photosynthetic irradiance enhances activities of certain pathogens on turf grown in shaded environments.
 
In particular, pythium blight, leaf spot, brown patch, and Dollar Spot had been identified as troublesome in low light environments.
 
Diseases are one of the most limiting factors in growing turf grasses under shady conditions.
 
Severity of any one disease was greater for grasses grown as a monoculture than for mixtures of different species.
 
Kentucky bluegrass was able persist better when included in a mixture with other species than when seeded alone.
 
Shade tolerant turfgrasses
 
 
Differences between cultivars have also been reported within cool-season turfgrass species.
 
Cool season shade mixtures - KB/FF/PR
Typically sun mixture would 60/20/20
shade mixture would 50/40/10
or can eliminate PR if desired.

Cultivar tolerance - remember there is typically intra species variability as there is interspecies variability.

 
Very important to get the latest shade tolerance rankings in your area for the grasses you want to plant. New varieties are being released every year. Any list I give you will soon be outdated. Sources of info TGIF, NTEP (USDA, Beltsville, MD)
 
Modification of shade environments
 
Light intensity reaching the turf can be increased by selectively pruning limbs within the tree canopy.
 
Selective pruning is particularly effective with trees such as maples and oaks that have dense canopies.
 
Sunflecks can be an important source of light for photosynthesis for turfs growing under shade.
 
Lower limbs of single trees should be prone to a height of 3 m.
 
This will allow for direct sunlight to reach the turf during early morning and late afternoon.
 
Dense shrub plantings should be thinned or selectively removed to prevent air stratification and improve wind movement.
 
This will decrease relative humidity in the turf environment and enhance drying.
 
Before establishing turf under trees, shallow feeder roots should be pruned to a depth of 10 cm.
 
Plant shade and ornamental trees that are deeper rooting with open canopies.
 
Fall establishment of cool-season grasses under deciduous trees is best.
 
Tree fertilization should be as deep as possible to discourage formation of shallow feeder roots.
 
Turfgrass cultural practices under shade
 
Mowing heights should be raised as high as possible to provide maximum leaf area for absorption of the limited radiant energy.
 
Increased mowing height increases the depth of turfgrass rooting and helps to maintain turf density.
 
Irrigation should be applied only when turf shows signs of stress.
 
Shallow, light, and frequent irrigation enhance disease activity and encourage development or shallow root systems.
 
Excessive N fertilization should be avoided.
 
This encourages shot growth over root growth which places a further stress on carbohydrate reserves.
 
Excessive N increases tissue succulence which increases disease susceptibility and decreases the ability of the turf to withstand environmental stress.
 
Minimize traffic in shaded areas since wear tolerance is reduced.
 
If all else fails
 
It is possible that no turfgrass species will provide a suitable groundcover in shade.
 
There are many alternative evergreen and deciduous ground cover species for deeply shaded environments.
 
In extreme shade, a mulch of bark, wood chips, or an inert mulching material such as pumice, rock or gravel maybe used in as a ground cover.