Trees vs Turf
Even though they both reside within the kingdom of plants, trees and grasses do not work and play well together. Trees over time will tend to dominate the landscape while the poor little grasses struggle to get their fair share of sunlight, water and nutrients.
If you have a mature landscape where trees and grass commingle, there are a number of things you can do to try and keep both happy. First though we need to get one thing straight...There is no such thing as 'shade proof' grass. There are excellent blends of shade tolerant grass seed (never sod...no sod does well in deep shade) but unless 2-3 hours of sunlight sneak through over the course of the day, the grass will not survive the summer.
Grass Can’t See the Light - Embrace Mulch Rings
That being said, you will want to prune and thin trees where called for to increase light penetration. Wherever we can't get at least 2-3 hours, eliminate the grass. Embrace mulch rings. It's common practice to have a 2-3 foot ring of mulch around any newly planted tree. It's less common to see folks increasing the size of the mulch ring as the tree gets bigger. A 10-12' mulch ring for a large tree is not out of line.
You Are Witness to a Literal Turf War
The roots of the tree directly compete with the turf for water, nutrients and space. Once you have a mulch ring sized appropriately for the tree(s), you may still find some tree roots popping up in the lawn. Some trees have terrible surface root problems (Silver Maple, Willow, Sweetgum to name a few). Most homeowners don't realize that you can prune a root just as you would prune a branch. It's true! You would use the same decision making process as pruning a branch. Compare the diameter of the root to be cut to the trunk. If it's less than 10% of the trunk diameter...cut away. If it is 10-20%, then think twice. Over 20% leave it be, unless you want to risk damaging the tree (Hey, that rhymes!)
As far as sharing available water, trees can be gluttons when it matters most...in the summer. Watering grass that's competing with tree roots in the summer can be tricky. The heat and humidity of summer creates prime conditions for lawn disease so you'll want to water early mornings so the grass leaves don't stay wet all night long. You don't have to water too often (once or twice per week) but when you do, give it a good soak.
Nutrient requirements will also be different for grass growing in the shade. First, the tree roots tend to deplete the soil of many key nutrients and, in many cases, make the soil more acidic. A separate soil test under the trees is not a bad idea. In addition, once the trees leaf out, the grass growing under the trees need very little nitrogen (the major component of lawn fertilizer). In fact, if you fertilize your grass in the shade at the same rate needed in full sun, you'll do more harm than good.
If You Wanted to Live in the Woods, You Would Have Moved to the Country
Of course most everyone wants a little grass with their trees because without grass, it's not a yard, it's the woods. So, if you think you're getting the prerequisite minimum sunlight, follow these tips and you'll be well on your way.
Struggling with grass in the shade, we've got a program for you! Check it out here.