GROWING GRASS UNDER TREES
Many people believe you can not grow grass under trees because there is not as much sunlight. Grasses do grow better in direct sunlight, however, they will grow in filtered light under trees. Basically, if you can read a book under the tree in the middle of the day then there is enough sunlight for the grass to survive.
The real culprit is not lack of sunlight but rather lack of water. During long dry spells the tree will take up all available water leaving none for your grass. If supplemental water is not added by irrigation, the grass will wither and die. So, if you want to grow grass under your trees it is necessary to put in an irrigation-system.
In addition to the irrigation you will need to fertilize the grass area under your trees more often than the rest of your yard because the tree will also steal all available nutrients leaving none for your grass. Adding extra fertilizer under your tree will provide additional nutrients for- your grass health.
The reason 'rye grass grows so well under your trees in the winter is because the trees are dormant in the winter. When the tree is not actively growing, it is not taking up all the moisture and nutrients.
GERMINATION OF GRASSES
Argentine , Pensacola , Tifton 9 , Bahiagrasses , Bermuda Grasses, Centipede, Zoysia and Millets all require. night temperatures above 65 degrees F to germinate. It does not matter if the temperature is 80 or 90 degrees during the day, if the temperature drops below 60 degrees at night the germination process slows down or completely shuts down. Ideally you need plenty of moisture and hot muggy nights above 70 degrees for these grasses to germinate in a couple of weeks. If you have a few warm muggy nights and then a cold front comes through dropping nights back down below 60 degrees the germination process will stop. These grasses do not want to germinate and take a chance of a possible frost burning their little new born leaf. So, nature keeps the embryo sleeping until it is sure spring has really sprung.
Rye grass works just the opposite. Nights need to be below 55 degrees with adequate moisture to stimulate germination.
WATER IS CRITICAL AFTER GERMINATION
It is very important that there is adequate moisture in the top one inch of soil for the first several weeks after germination. The little grass seedling will 'try desperately to grow it's roots down as quickly as possible to prevent dehydration. Irrigation is the only way to ensure enough-moisture to keep the seedling alive if there is not adequate rainfall.