Shade grass refers to any type of grass or combination of grass varieties that are shade tolerant.
Shade can range from partial shade (some sun during the day) to full shade (shady most of the day). Shade can also refer to dappled shade (light penetrating shady tree branches, and light or heavy shade (under varying tree canopies). Some grasses do better in varying degrees of shade.
Common grasses like Kentucky bluegrass or perennial ryegrass are not shade lovers. They thrive is full sun or moderate sun and seed blends heavy in these types of grass are often the reason grass does not seem to grow in the shade. The best cool season shade grasses are from the fescue family. Standard grass seed blends usually contain about 1/3 fescue and between the three varieties, and dominant species will emerge depending largely on the amount of sunlight it receives. Very shady areas will likely need a custom seed blend for shady areas.
Creeping red fescue is the best performer but is often blended with Hard fescue and Chewings fescue to easier adapt to varying degrees of shade and different soil types. Tall fescues also do well in the shade and it is not uncommon to find it in seed blends specialized for shade.
Each fescue variety contains many different cultivars, some preferred for their disease resistance, drought tolerance or soil adaptability. High profile cultivars will ramp up the cost and are mainly used in high-end applications like golf courses. A mid-priced cultivar should be fine for most shady lawns. Cheaper seed will likely result in lower germination rates and disappointment.
Fescues and endophytes
Fescues are among the rare turfgrasses that are able to host endophytes. Endophytes in grasses are a type of fungus that live symbiotically with the plant. The endophytes do not harm the grass in fact their presence has been found to be beneficial to the health of the turf. The presence of endophytes enables the grass to better withstand stresses like heat and drought, and provide an element of insect and mammal resistance. Endophytes are naturally occurring in some occasions but grass seed can also be inoculated after harvesting and are a safe, natural way to provide another level of defense against diseases, pests, and other plant stresses. Endophyte inoculated seed needs to be stored in a cool, dry environment or the benefits will be greatly reduced so it is important to order seed from a reputable source using fresh stock.
Warm season shade grass
The best warm season shade grass is St. Augustine grass, but it cannot be bought as seed. It must be sprigged or planted as sod. Zoysia grass and Centipede grass are also decent shade grasses for southern climates, however, the more northern the lawn, the less these grasses will thrive in the shade. Closer to the transition zone, and including the transition zone, fine fescues are more suitable for shade tolerance.
Under a Shady Tree
If trying to grow grass under a tree, keep in mind that the new grass will be, not only lacking in sunlight but in direct competition with the trees for water and nutrients. Ensure the is adequate soil to sustain the grass among the tree roots. The grass will likely need help by way of extra watering and fertilizing. It would also help if the tree or trees could be thinned out by pruning to allow as much available sunlight as possible to penetrate the canopy.
Sometimes, the reason grass doesn't want to grow in a shady area is because it wasn't meant to. It might be necessary to think outside the box when dealing with shady areas. If grass doesn't seem to grow under a tree, perhaps a mulched bed could be an option. Ground covers like pachysandra also make acceptable alternatives to planting grass. Shade loving annuals could be planted under trees or left undisturbed and completely natural.