After last week’s Gardening Tip was posted, I had a few people ask me to provide some follow-up information so let’s try to address each of these in today’s article:

• Can we to run the lawn mower over the leaves?
• Can I leave my leaves on my lawn?
• Coming soon: Leaf blowers – good or bad?

Can we to run the lawn mower over the leaves?  First let’s talk briefly about grass clippings.  You can use a regular or mulching lawn mower to on grass clippings. The mulching lawn mower is better as it shreds grass clippings into tiny bits, allowing it to slowly turn into a compost that is rich in nutrients such as nitrogen. You can do this throughout the summer and into the warm part of autumn. If your grass was overly long when you cut it, or if the lawn was wet, then you should consider composting them in a compost bin or bagging them yard waste bags. Another time to not leave grass clippings is if your yard appears to have bare spots. This may be a sign of disease and you don’t want to leave the clippings nor add them to your compost bin possibly spreading the disease.

For leaves, you can use the mulching lawn mower or if you don’t have one, take the bag off your lawn mower, set the blade up as high as it can go, then run over fallen leaves with your lawn mower a few times. You'll know you're done when about half an inch of grass can be seen through the mulched leaf layer. If you can’t see any grass – then reattach the bag and go over the lawn again.

The leaf bits in the bag can be spread over your garden areas or added to your compost bin. Once the leaf bits settle into the lawn, microbes and worms get to work recycling them.  Many experts suggest that after you mulch your leaves, feed the lawn with some nitrogen rich lawn fertilizer to help speed the composting process. 

This leaf “mulch” can be left on the lawn but be sure you are not leaving a large layer of it. as long as there is not more than a 1” layer of mulch on the lawn. More than that can cause harm.  Chemically speaking, when there is too much leaf mulch microbes in non-composted leaves may compete with grass for nitrogen during the decomposition process, lessening the beneficial effects of the mulch.  If you have more than a 1-inch layer of mulch on the lawn, you need to stop leaving the chopped-up leaves and/or grass clippings on your lawn. Put them into a compost bin so they turn into compost for next year. And in the fall, put the fallen leaves onto your garden beds.

Can I leave my leaves on my lawn?  People may be concerned that the chemicals, such as tannins, in leaves will have a negative impact on our lawns.  The tannins that help make good red wine and bright fall colors (in leaves) may also help make good soil and healthy livestock, according to a multi-year research effort between the US Department of Agriculture (USDA), it’s Agricultural Research Service (ARS), and the tannin laboratory in Miami University. The teams combined their efforts and capabilities to identify important features of tannins that determine soil interactions. Below are some of their findings.¹

Tannins and other organic compounds are common in many plants which enter soil from plant roots and decaying leaves. They may become part of the rain or melting snow as it runs down tree bark or drips off the leaves.

Results show that the rate and amount of binding to soil varied from one tannin to another meaning that different tannins will have more or less impact than others. Scientists also found that tannins can immobilize heavy metals which will lessen their toxic effect on root growth. (good news) And tannins and related organic compounds can free up nutrients such as calcium for crop use. (good news) Scientists hope to learn more about how forest soil microbes metabolize tannins, leading to a better understanding of the multiple ecological functions performed by tannins. At this point, tannins do have some benefits but be advised not to leave too many on your lawn and ensure you mulch them.  Moving the leaves into your gardens can have a positive composting impact. And putting excess leaves into a compost bin is a great idea to generate compost for the next season.

Reference 1. USDA Agricultural Research Service’s article “Tannins Surprising Benefits for Soils, Forests, and Farms. Link: https://www.ars.usda.gov/news-events/news/research-news/2010/tannins-surprising-benefits-for-soils-forests-and-farms

Article by Doreen Coyne, a member of the Richmond Hill Garden & Horticultural Society