I have hundreds of mature trees in my back yard. Some areas I keep the grass neatly trimmed right up to the trees but some trees are near river so I let the grass grow tall all around them. Which is better for the trees?
Lucky you to have many mature trees, and along a river too! The days of fine, green lawns rolling between majestic trees should have gone out when our forebears left the lush gardens of Europe, as well as the multitude of gardeners needed to keep them that way. Of course, that didn’t happen, as we still have gardens and parks following the model borrowed from Europe all those years ago.
Today, we understand the needs of trees a little bit better, especially when adjacent to a natural feature, such as a river. Even though your trees are mature, their feeder roots are still close to the surface and would benefit from denser cover than mowed lawn. Short grass, especially during drought conditions, allows rain to roll away, and in your case, down to the river. If you use fertilizer on your lawn, that gets carried to the river, and the extra nutrients can be detrimental to the wildlife there. Longer grasses and other cover tend to hold the moisture in place, allowing it to soak in where it’s needed. They also don’t need fertilizer.
As well, if you use a whipper snipper, the high speed of the nylon cutter can damage bark, when used close to the tree. This is especially true for young, tender bark, but can also damage bark on mature trees, which in turn, can invite infection.
A further consideration when having a grove of trees near a river is the invitation to so much more. By leaving the natural vegetation between the trees, the seeds or fruit from those trees can be left in place for both food for wildlife, and new trees to grow. Those mature trees should be sheltering young trees for the future.
Letting the grass grow will allow a more diversity of plants, as the wind and the wildlife bring in other seeds. The more diversity there is in the plant life, the more diverse is the wildlife, and therefore a healthier habitat – including healthier trees. You may have found that the grass and plants you let grow near the river has attracted other wildlife. The shade from the trees and the plants adjacent to the river casts shadow over the water – this provides cover for young fish and other aquatic creatures.
That is not to day the area can’t be managed. Non-native or invasive trees, shrubs or plants can be removed. A path to the river can be maintained between the trees, simply by keeping that swath mowed.
So, the short answer: it is better to let the grass grow- it’s better for the trees.