Fall Leaves Can Harm Your Garden

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Robert Pavlis

Fall leaves are a great resource of nutrients for the garden but if you use them incorrectly, they can rob your soil of nitrogen and make it difficult for plants to grow. This is especially true in a vegetable garden where you are trying to grow things quickly from seed.

In this post I discuss the decomposition of fall leaves and explain how to properly use them in the garden. In the process you might discover some interesting things about your compost pile.

Fall Leaves can Harm Your Garden
Fall Leaves Can Harm Your Garden

How to Use Fall Leaves

What should you do with fall leaves?

Take Them to the Curb

From a financial position and an environmental position this option makes no sense. The leaves are a great addition to your garden and there is no point in giving them away.

Leave Them on the Lawn

Lots of people are now saying this is a good idea, but even if you shred them first with a mower, too many leaves on the lawn will kill your grass. This is only a good idea if you do it in small quantities.

Food Science for Gardeners, by Robert Pavlis

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Cover Garden Beds With the Leaves

This works great for most plants. The leaves keep the crown of perennials warmer, and help collect snow when it does fall. Provided the leaves are not too thick they can just be left alone in spring and they will decompose. If you have thick layers, say more than one inch, you will want to remove some in spring as the plants start to grow again.

Some plants, like small alpines, do not want to be wet in winter and a covering of leaves might cause them to rot.

Add Them to the Mulch Pile

Another great idea that turns the leaves into future nutrients for the garden.

Make Leaf Mold

As I will discuss below, fall leaves have a high carbon level which slows down their decomposition. If you simply pile up the leaves on their own, without adding extra high nitrogen material, the fungi will slowly decompose the leaves into something called leaf mold. In a year or two this will make a great mulch for the garden.

Bury Them In the Garden

This is a popular approach by people who like to dig in their garden. They cover it with leaves, and then dig or rototill the whole thing so that the leaves are covered with soil. Some people do this in a perennial garden – I’m not sure how without damaging a lot of roots – but it works best in a vegetable garden because most of the plants have been removed during fall clean up.

On the surface this sounds like a great idea. You are adding organic matter to the soil, which will decompose and release nutrients for future crops. The reality is different.

Decomposition of Fall Leaves

The key to this process is the C:N ratio, which is the relative amount of carbon to nitrogen. Microbes are most efficient with a ratio of 30:1, which is the number usually recommended for making fast compost.

Plant Science for Gardeners by Robert Pavlis

The C:N ratio of leaves will depend on environmental conditions as well as the tree species, but the range for fall leaves is in the order 35-85:1. There is approximately twice as much carbon as nitrogen. In order for microbes to decompose the leaves, they need to find another nitrogen source, which is usually the soil.

If the leaves are used as a mulch, the nitrogen is taken from the top few millimeters of soil which won’t affect plant growth. But when the leaves are buried in the soil, the nitrogen is taken from soil next to each leaf. If this is next to a growing plant, the microbes will compete with the plant for nitrogen and the plant usually suffers.

As the decomposition process continues, the C:N ratio declines and once it reaches approximately 17:1, excess nitrogen becomes available to plants. When decomposition is complete, the ratio will be 10:1.

Burying fall leaves in the garden can result in nitrogen deficiencies in plants the following spring and summer. The degree of this deficiency depends on the amount of available nitrogen in the soil and the amount of leaves. One way to overcome this problem is to add nitrogen fertilizer to the garden in the spring which will meet the needs of both the microbes and the plants.

Its better not to bury them; use them as a mulch instead. This will prevent a nitrogen deficiency and preserve soil structure because you aren’t disturbing the soil.

Nutrients in Fall Leaves

What kind of nutrients do fall leaves add to your garden? Spectrum Analytic Inc analysed 100 municipal leaf samples collected from across New Jersey and found the nutrient values in the table below. The average NPK value would be 1-0.2-0.5. They also add many of the micronutrients, which should not be surprising since all plant material have about the same amount of these.

 

Nutrients in Fall Leaves, by Spectrum Analytic Inc
Nutrients in Fall Leaves, by Spectrum Analytic Inc

References:

  1. Nitrogen as a Nutrient; http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1130447042&topicorder=2&maxto=8
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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

41 thoughts on “Fall Leaves Can Harm Your Garden”

  1. I have been experimenting with decomposing leaves to create soil, for a couple years I layered clay soil with leaves and it breaks down in about 18 months. This year I chopped the leaves in a shredder and layered them with soil and a little feather meal and I might see them broken down quicker. I haven’t seen any set formula for doing this but with leaves available for free it seems like a good idea to enable people to create their own soil.

    Reply
    • Leaves won’t create soil, but they do add organic matter to soil.
      Be careful if you plant in soil that has buried leaves – they do suck out nitrogen from the soil as they decompose.

      Reply

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