I got another interesting question recently. We handle tree care as well as lawn care, and you’d be surprised how much overlap there is.
“Spruce needles (lots of reddish brown needles) have fallen from very old trees onto my lawn, the trees cannot be saved, but what do I do with the lawn? Can I leave the needles or do I have to remove them?”
Evergreens drop some needles annually as part of their growing process, especially when they are old or sick. This is a problem for turfgrass because when the needles break down, they cause a “matting effect” in the soil bed surrounding the tree causing difficulty for turf (or most plants for that matter) to grow well. “Matting” means that the needles, which are slow to decompose, are layered with pockets of air and no real soil, leaving nothing to properly root into. In addition, they can contribute to making the soil more acidic.
Once the pH of the soil starts going down, it is very difficult for the grass to absorb nutrients. That’s one of the main reasons why you never see really healthy grass underneath pines and spruces.
You can improve the situation with regular limings – that will help bring the pH back up. you can also try to rake up the needles to reduce the matting effect, and adding a small amount of organic matter by way of topdressing to help. But you need to remember, growing grass under the canopy and shade of any mature tree, and competing with its root system is really difficult if not impossible. You will never get the grass under your evergreens to look as nice as the rest of your lawn.
Another fix would be to find a more suitable landscape idea for underneath trees, try just mulching under the base and the outer perimeter of your trees. That way you could have thick, lush grass up to the drip-line of the trees with a clean transition to attractive wood mulch. Much nicer than your lawn just petering out in sickly-looking tufts.
**Another point to remember: Poor fertilization choices can cause soil to become more acidic over time, so it very important to know your soil and choose the right fertility program for your lawn- not just any off the shelf package with good marketing!