Can You Mulch Around Onions? (Plus Best Mulches to Use)

Are you annoyed by the constant need to pull weeds and water your onion plants? If so, you are probably thinking about putting down a layer of mulch around your onions, and wondering if it will hurt them.

So, can you mulch around onions? You can use mulch around onions, as long as you don’t bury them too deep. Use only 2 to 4 inches (5 to 10 centimeters) of mulch to avoid smothering the plants with too much heat or too little air. Also, wait to use mulch until the onions grow a bit larger. That way, you won’t bury the small seedlings.

Of course, there are lots of materials you can use to mulch around onions.

If you want to get some ideas for what to use for mulching onions, then you’ve come to the right place.

Let’s get started.

Can You Mulch Around Onions?

You can mulch around onions, but go easy on the mulch! Mulch is not the same as soil, and you don’t want to bury onions in too much of either.

You only need 2 to 4 inches (5 to 10 centimeters) of mulch, depending on the material you use.

If you bury the onions under too much mulch, the soil may stay too wet after heavy rain. A thick layer of mulch may also retain too much heat in the ground during hot weather.

Why to Mulch Around Onions

As long as you use a reasonable amount of mulch, it can benefit your onions in several ways.

Weed Suppression

Putting a layer of mulch around your onions will suppress weeds by preventing their seeds from growing. Also, a layer of mulch applied over growing weeds will smother them due to lack of sunlight and air.

When your onion plants do not have to compete with weeds, they will get more water, nutrients, and sunlight. This will give you healthier plants with larger onion bulbs at the end of the season.

To learn just how big your onion bulbs can get, check out the article I wrote about it here.

You can also learn some tricks to get bigger, better onions in my article here.

Water Retention

A layer of mulch around onions also helps to retain moisture in the soil. This helps your plants to survive an extended hot, dry spell.

sandy soil — Mulch can help retain water, which is helpful in areas with dry soil and little rainfall.

It also decreases the need to water your onion plants during the season. This is helpful in areas where water restrictions or bans are in effect, when every little bit of water counts.

Soil Insulation

Mulching around onions insulates the soil against temperature changes. This helps to avoid cold damage to onions if you get a hard frost in late spring.

Onions can tolerate a light frost, but seedlings may succumb to harder freezes. If you are worried about cold damage to onion plants, you can try using cloches to protect them.

frosted leaf — Onions can tolerate light frost, but a hard freeze will hurt them.

A cloche is a cover used to protect plants from cold and pests. A cloche is often made of plastic, and it sometimes has a vent you can open to allow air to circulate.

You can make a cloche if you cut the bottom out of a clear plastic bottle and put it over a plant. On a hot day, you can remove the bottle cap to vent out excess heat and keep the plant happy.

Extra Nutrients

One of the best parts about mulch is that it will decompose over time. After mulch decomposes, it will add nutrients to the soil.

Mulch also adds organic material if you use natural sources, such as compost, leaves, or grass. Of course, it takes time for mulch to decompose, depending on what type you use.

Best Materials for Mulching Around Onions

In general, you should use what you have available as mulch. The only exception is if you think the material might contain pesticides or herbicides.

The problem with pesticides is that they may hurt bees and other beneficial insects in your garden. Some pesticides do not discriminate based on the type of insect and will hurt them all.

The problem with herbicides is that they can destroy crops you want to grow, not just weeds. If a horse ate grass that contained herbicides and you use the resulting horse manure, then you might be introducing herbicides to your garden.

With those warnings out of the way, let’s take a look at some of the best materials for mulching around onions. We’ll start with a common one that is easy to find: grass clippings.

Grass Clippings

You can get lots of grass clippings every week if you have a big lawn to mow. You can also ask your neighbors for clippings.

You can even ask a local landscaping company to drop off their grass clippings when they are in your neighborhood. They might be happy to do so, since they probably have to go out of their way to drop off the clippings at a local compost site.

cutting grass — Grass clippings are common and easy to find, and they make a good mulch for onions.

No matter what your source of grass clippings, heed the warning above about pesticides and herbicides used on lawns!

Another big benefit of grass clippings is that they will add lots of nitrogen to your garden as they decompose.

If you have extra grass clippings, you can also turn them into compost for your garden. For more information, check out my article on how to compost grass clippings.

When mulching with grass clippings, the University of Minnesota Extension suggests using a layer of mulch 3 to 4 inches (7.5 to 10 centimeters) deep.

Leaves

After fall cleanup, you might have lots of raked leaves lying around. Your neighbors might have a bunch too.

Instead of letting them go to waste, you can use them to mulch your onions. Leaves can be used whole, or you can chop them up into smaller pieces before spreading a layer of them in the garden.

compost bin — You can turn leaves into compost, or use them as mulch for your onions.

As with grass, you can also turn leaves into compost. You can learn more about how to make compost from leaves and other yard waste in my article here.

Sawdust

If you know a carpenter or someone who has a sawmill, then you have a nice source of sawdust for your garden.

Always use a clean source of sawdust. Make sure that it comes from wood that is not treated with chemicals, such as:

CCA (chromated copper arsenate) – used in older pressure-treated wood
Creosote – used to treat railroad ties

Also, be careful about adding too much sawdust if you use it as mulch. Sawdust contains a lot of carbon, and this carbon will tie up nitrogen in the soil.

This makes nitrogen unavailable to your onions, which will hurt their growth. To avoid this problem, do not mix sawdust directly into your soil.

Instead, use it on top of the soil as a thin layer of mulch. Another option is to turn sawdust into compost first before mixing it into the soil.

To learn how to do this, check out my article on how to compost sawdust.

Newspaper

If you collect enough newspapers from friends and neighbors, you can use them to cover the ground around your onions. This will help to prevent weeds, just like any other mulch.

To kill existing weeds, you will need a thicker layer of newspaper. If you cannot get enough newspaper to use as mulch, consider cardboard as an alternative.

Cardboard

Cardboard is more heavy-duty than newspaper, so it will take a little longer to break down. Cardboard is great for suppressing new weeds and smothering existing weeds.

Cardboard is both effective and plentiful, making it an economical choice for mulching onions. You can sometimes find cardboard boxes free for the taking at wholesale clubs such as Costco or BJ’s.

Wood Chips

When people hear the word ‘mulch’, they often think of wood chips first. Wood chips take a lot longer to decompose than grass or leaves, which is good if you don’t want to apply mulch often.

wood chip mulch — Wood chips will last longer than grass or leaves as mulch.

The heartwood of redwood, cedar, and cypress all contain natural oils that help them to resist decay, so they last the longest of all. However, wood chips made from these trees will be more expensive.

Keep in mind that wood chips are more often used as a landscaping feature, rather than in a vegetable garden.

Pine Needles

Using pine needles as mulch is an option if you have pine trees on your property. This is a good way to reuse the needles after you clean them up.

Compost

You can use yard waste and kitchen scraps to make compost. You can then mix the compost into your garden soil.

This adds nutrients to the soil, helping onions and other plants to grow better. It also adds organic material to the soil, which makes clay drain better.

However, you can also spread compost on top of soil to act like any other mulch. The only problem is that it may actually encourage weeds to grow, since it contains nutrients and organic material for them.

Manure

Manure is simply animal waste and bedding. You can use chicken, cow, or horse manure as mulch for your garden.

However, make sure the manure is aged several months before using it in your garden. For one thing, manure that is too fresh will be high in nitrogen (“hot”).

This can burn your plants with too much nitrogen all at once. Also, fresh manure may still contain pathogens, which can make you sick if you grow vegetables like onions, potatoes, carrots, and others that come in contact with the soil.

One downside is that some manure may also contain undigested seeds from weeds. These may grow in your garden later in the season or years from now.

Straw

Straw comes from the leftover part of growing grain (such as wheat) for food. It does not contain the seeds of grass or weeds, which makes it a good choice for mulch.

On the other hand, hay comes from grass, and it contains seeds that could sprout in your garden if you use it as mulch.

Biodegradable Mulches

There are also some man-made mulch options that will biodegrade (break down naturally over time). Most of these are made from plant fibers, such as wood pulp.

For example, you could try this paper mulch from Gardener’s Supply Company.

You might also want to check out this Dewitt Biodegradable Weed Barrier from Greenhouse Megastore.

When to Mulch Onions

You should mulch onion plants in the spring after planting them. However, give your onion seedlings time to grow into more mature plants before applying mulch.

onion seedling sprouts — These onion seedlings are a little small to mulch. Give them some time to grow first.

Otherwise, they can get buried below the mulch. This may keep them too wet, leading to rot. It might also make them too hot, or reduce air flow.

Sprouted onions are much larger and more established than seedlings. In that case, you don’t need to wait to mulch them.

If you’re curious about planting sprouted onions from your kitchen, you can learn more about how to do it in my article here.

How to Mulch Around Onions

Once you have chosen a type of mulch to use, be sure that you get it from a clean source. For example, avoid grass clippings from a lawn grown with pesticides or herbicides.

If the soil around your onions is dry, go ahead and water before mulching.

Spread a layer of mulch 2 to 4 inches (5 to 10 centimeters) thick around your onions. Spread it out as far as you need to prevent weeds from growing near your onions.

Avoid the temptation to put mulch directly against the onions. Leave a small circle of bare soil right around the onion itself.

In the fall, mix grass clippings or leaves into the soil if you used them as mulch. They will continue to decompose over the winter and into the next spring.

If you want to try something a little different, you can experiment with a deeper layer of mulch. The Prairie Homestead has an article on the deep mulch method, which you can read here.