Gathered grass clippings will certainly have some grass and weed seed in them. Some of that seed will survive the natural heating that occurs when clippings are allowed to sit for several days before use in the garden. So when you’re carefully mulching your crops, you’re probably importing grass and weed seed into your garden at the same time. While that sounds really bad, continued mulching usually takes care of such weed seed.
Using grass clipping mulch is pretty straightforward, providing one follows a couple of cautions. You cannot use grass clippings from a lawn that has been treated for weeds. The herbicides applied for lawn weed control may/will carry over in the clippings and damage your garden plants, if not kill them outright. Secondly, you have to be careful using fresh, wet, green grass clippings around tender young plants. Just as a pile of grass left on the lawn heats up as it decays and damages or kills the grass under it, fresh clippings applied in an inch or more layer will heat up, "burning" any plant they’re touching or near. Of course, if you’re laying the grass clippings over weed seedlings, such heating and burning can be a very good thing.
Insects can hide in mulch. We mulch our pumpkins and butternut squash, but have to spray more, as squash bugs seem to love grass clipping mulch. Also, mulch left on the ground through the winter can provide protected areas for insects and their eggs to overwinter.
Note that not all "experts" agree that using grass clippings for garden mulch is a good practice. Some suggest using a mulching mower to return the clippings’ nutrients to ones lawn as a better option.
Alternatively, one can let grass clippings sit on the lawn for a day to dry if not piled too deep before raking or sweeping. That allows the grass to dry and cure a bit, allowing their use close to plants as soon as they’re gathered. The downside to this practice is that if the clippings are too thick and significantly heat up overnight, they may leave a row of damaged, or worse yet, dead, grass underneath where they laid.
That’s the easy part. Using a leaf rake to rake down a row of clippings is a whole lot easier than having to rake an entire lawn. But it’s still hard work. If you mow a lot of ground with a riding mower as we do, a lawn sweeper becomes a necessity, rather than a luxury. At least, that’s what I tell my wife. Your mileage may vary in selling that tale.
If you have a mower with a bagger, you’re already in business for gathering your grass clippings. If not, you’ll want to change your mowing practice a bit to produce small windrows of clippings much like a farmer does when raking hay. To do so, first mow several rows clear and then reverse your mowing direction to blow the clippings towards the cleared patch. Repeat the process, leaving long rows of clippings windrowed across your lawn.
The idea behind this misconception is that by removing the clippings and weed seedheads, I will be reducing the number of weeds in my lawn for next year. This may seem like an intuitive strategy for reducing weeds in a lawn over time, but the research doesn’t support this practice as being effective.
By maintaining your lawn at a taller height, you will have a lawn that requires less watering, and is more heat, insect, and disease tolerant throughout the growing season. Finally, by mowing according to the 1/3rd rule, you will also have to mow your lawn less frequently throughout the year.
Misconception #2: Collecting or bagging lawn clippings will reduce the amount of thatch in my lawn.
As an example, when mowing your lawn at 3.0 inches, you would want to mow the lawn when it gets to 4.5 inches (removing 1.5 inches of turf is equal to 1/3rd of the lawn height at 4.5 inches), which would occur about every 7 days for most of Minnesota. But when mowing your lawn at 4.0 inches, you would want to mow the lawn when it gets to a height of 6.0 inches, which would occur about every 10-12 days.
Furthermore, by removing lawn clippings from the lawn, you are removing up to 2.0 lbs N / 1000 ft2 per year that the lawn would use to be thicken and reduce weed pressure over time. After all, a dense, healthy turf is the best defense against weeds.
Some species, such as Kentucky bluegrass, spread by rhizomes and grow a little more aggressively, naturally creating a thatch-mat layer. Other species, such as tall fescue or the fine fescues, do not readily create a thatch-mat layer.