Some gardeners discourage using oak leaves in compost, and while there’s good reason to be cautious about when and how you use them, oak leaves aren’t necessarily bad to add to your compost heap.
Compost is usually added to gardens and lawns to provide fertilizer, build soil, and buffer the pH level since most compost is just slightly acidic, which is the preference for many plants. Oak leaves have a lower pH than average, so compost with large pieces of oak leaves in it will lower a soil’s pH level, but when the material is completely broken down, it becomes almost neutral and can be used in any situation.
What Leaves Should Not Be Composted?
In nature, all leaves become soil, so pretty much any leaves you’ll find on your property can be composted, but you may have to pay extra attention to be sure they break down completely before you apply the finished product. For example, poison ivy, a woody vine, might not break down as quickly as herbaceous weeds. If the root of this vine is still present when you use the compost, it might still be able to grow back if you spread it out before it’s broken down enough.
This is because roots and other carbon-rich woody materials take much longer to break down than green, nitrogen-based materials like leaves, stems, and even seeds. During the composting process, especially if you hot compost, good bacteria break down the green material at temperatures that destroy pathogens and compounds that might become a problem if they weren’t broken down all the way before spreading across your lawn or garden.
Are Oak Leaves OK For Compost?
Oak leaves are great to add to your compost, but there are a few properties about this leaf that require a little extra attention in some cases:
- Oak leaves are acidic in nature
- Oak leaves take longer than average to break down
- Many plants prefer acidic soil
Oak Leaves Are Acidic In Nature
While oak leaves have a low pH level of 4.5, once they’re broken down into soil, the acidity gets neutralized. This means that compost with lots of oak leaves in it will be on the acidic side because the dead leaf material still contains an above-average level of tannins, a bitter compound found in some trees, shrubs, and fruits meant to protect against being eaten by animals.
Since compost usually still contains small pieces of leaf matter (among other woody and carbon-rich things like twigs, bark, and stems), the tannins are released into the soil and lower the pH.
Oak Leaves Take Longer Than Average To Break Down
Oak leaves contain more lignin than other trees, meaning their leaves are denser, waxier, and take longer to break down than, for example, maple leaves, which are much thinner with less lignin content. While dead leaves usually take a few months to break down, dead oak leaves take up to 8 months to decompose fully.
This gives them staying power as a mulch, but if planted around trees, flowers, or gardens that don’t like acidic soil, it can damage the roots in that soil. A low soil pH frees up higher levels of nutrients that become toxic in large quantities, like iron, and makes other nutrients unavailable, like calcium and potassium.
Certain Plants Prefer Acidic Soil
If you’re making compost for acid-loving plants like the oaks you got the leaves from, dogwoods, hydrangeas, or blueberries, then making an ericaceous, or acidic, compost intentionally will help support those plants’ growth in a way normal compost can’t.
When Should I Use Oak Leaves For Compost?
The end use for your compost will determine whether you should use your oak leaves in one or another compost pile. If you only have one compost heap going, and it’s intended for plants that like slightly acidic to neutral soil, it might be best to start a new pile with your oak leaves.
Before applying any soil amendments, be sure you know which kind of soil your plants prefer, and check the pH before applying. Some of the situations in which an oak-heavy compost is a good choice include:
- Mulching around your oak trees
- Amending soil under acid-loving plants
- When they’re part of a varied mix of materials
Mulching Around Your Oak Trees
If you have oak leaves, chances are you have oak trees as well. In nature, the soil under oak trees is built up over time by their fallen leaves, so they’re well-accustomed to the lower pH level of soil. Leaves provide all the essential nutrients that their parent trees need, so using oak leaf compost or mulch at the base of the tree is one of the best uses for your fallen oak leaves.
Amending Soil Under Acid-Loving Plants
If you grow other acid-loving plants like azaleas, raspberries, holly, pumpkins, or leeks, an oak compost or mulch is a great ground cover that maintains moisture, provides nutrients, and conditions the soil’s pH for an optimal growing environment.
When They’re Part Of A Varied Mix Of Materials
A well-mixed compost made of many kinds of leaves and materials will dilute the effects of the oak tannins. They’ll be neutralized over time, and if the leaves aren’t in large quantities, the acids will be diluted and likely won’t be a problem.
Always check the pH of compost before you apply it; to be sure it’s in the range preferred by the plants you’re providing it to. If you allow the compost to break down over 8 months, the resulting compost might be neutral enough to use in any situation.
How To Make An Oak Leaf Compost
After raking up the oak leaves on your lawn, find a place to make your compost heap. This could be an open pile in the far back of your yard or in a closed bin or tumbler. In either case, mow over the leaves with your lawn mower, using a mulching blade if you can, to get the pieces as small as you can. This helps them break down faster and create more even compost.
The pile should be given some moisture (to make it about as wet as a wrung-out sponge) and aerated once every 5 to 7 days to make sure it’s well-mixed and oxygenated. Since oak leaves are mostly carbon, you should add about as many grass clippings and food scraps (green material) as there are dead leaves (brown material) to have a good balance of nitrogen to carbon.
Oak leaves can take up to 8 months to completely break down, making this compost a good ground cover after about 6 weeks of decomposition. Check the consistency of the compost before you put it down to be sure it’s ready (it should be dark, almost black, smooth, and moist, with some pieces of carbon-rich organic matter like leaves, swigs, or stems) and that the pH level is appropriate for where you’ll be applying it.