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.
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.
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 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 cleanup.
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.
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 of 35-85:1. That is way too much carbon. In order for microbes to decompose the leaves, they need to find another nitrogen source, which is usually taken from 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 the 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.
It’s 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. analyzed 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 has about the same amount of these.
References:
- Nitrogen as a Nutrient; http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1130447042&topicorder=2&maxto=8
Hello,
Thank you for this article, it explained things very clearly. And saved my vegetable patch Fromm a planned burial if dead leaves.
After reading other blog posts, I was going to bury my fall leaves in my vegetable patch. What’s to lose, I thought, lots extra nutrients for my future veggies! But the scientific explanation you provided made me understand the nuances of recommendations I had read before.
Thank you, you helped me in my continuous journey to be a better gardener.
Your welcome.
I been adding dried oak leaves in the planting hole for years with alfalfa pellets then I put soil of a couple inches over two items. then plant goes right on top . No issues !! You don’t need to be a scientific genius to know . Sometimes people over think that the more you think you know you just got to give it a try
You are forgetting that the alfalfa pellets are high in nitrogen, so they compensate for the low nitrogen level of the oak leaves.
Science is what explains why this works.
I’ve been heavily mulching my garden with leaves, some mulched some not, for three years. This year nothing is growing. Now I know why. Would blood meal work to bring the nitrogen back up?
yes
Hi! Can I put last Fall’s aspen leaves in my grow boxes now? Due to surgery last fall I could not get them raked and put on my grow beds but the snow is leaving and I’m raking them up. they are black but I’m wondering if I can still mulch them in? Thank you so much!
Use them as mulch.
Hello, I had to do extensive digging in the yard for drainage this last year. I left the falls leaves on the exposed soil to keep the soil from washing down the drain with the spring rain. This spring I had planned to roto till the leaves into the very clayey soil) and then lie down crass seed. Is this a good or a bad idea?
That will work – add extra nitrogen to help decompose the leaves.
Okay damage has been done. In November my husband chopped up and bagged leaves to put on my tomato containers. He has been working them into the soil all winter. So NOW I find out this is bad. What kind of slow release nitrogen fertilizer show I use? They are 25 gallon tubs. Can I use, a couple of cups, of alfalfa pellets? I am about to lose my mind. We have 2 months until plant time.
Just add some extra nitrogen fertilizer and they will be fine.
Help! We may have messed up our garden while preparing it for winter. Idk but we put a layer of unmulched leaves (not a heavy dose, just enough to cover the ground) then topped with aged manure so they wouldn’t blow away. Sounds like we should have put the leaves on top? Any suggestions?
Provided it is not on the lawn, it is not too much manure and the leaf layer is not too thick, there is no problem putting the manure on top.
Thank you for this post, exactly what I was looking for. I covered my raised vegetable beds with a layer of birch leaves. In order to make sure they don’t get blown off Im wondering if I should cover them with a layer of plant fleece. (the white layer that sometimes gets used to protect really young plants in the spring) Any clue if that’s a good idea?
I find that they mostly stay in place and do not need to be covered. But that may depend on your wind.
Thank you for this post. I reference it frequently. I’m in Nebraska and most of the state is in a drought, hardly any winter snowfall. At least a foot behind average. I put mulched leaves on my garden late fall every year. This year, not only has more of it blown away, but it also seems like they’re much less broken down right now that normal. It really looks like I put them on just yesterday. I could just be impatient, but I’m wondering if the lack of snow/moisture has slowed down the decomposition. On the other hand, I thought the warmer than average weather might encourage more of the little critters to get eatin, speeding it up. Do you have any thoughts on that? Would it be useful or just a waste to “water” the leaves somewhat?
dry leaves will decompose more slowly than wet ones. Bacteria and fungi need water.
Hi i have picked up the leaves from my lawn and put them on the soil around the perimeter of my garden where various plants/ shrubs are planted. Its a very thin layer of leaves. We are in winter at moment. Can i leave them till spring or should i pick them up and mulch them and then put them on?
I just leave mine.