Which Mulch To Use Where (and Why)

Mulching is a great idea in a polytunnel garden. It can help to retain soil moisture and reduce watering needs. It can often add fertility too. And in certain situations, a heavy mulch can also help to keep down weeds and reduce your workload as a gardener. But it is important to understand which mulch to use where (and why). The materials you choose to mulch with will have a bearing on the results you are able to achieve.

It is important to think about the characteristics of the mulch you are considering. You need to match up those characteristics with the needs of the plants around which it will be placed. In this article, we’ll take a look at some common mulches used for different reasons. This should help you work out where to use them in your polytunnel (and why).

Carbon Rich Organic Mulches

The first category of organic mulches that we will look at is those that are high in carbon content. These are often used to create a longer-lasting barrier between plants and soil. The material will break down – but only relatively slowly compared to other mulch materials. Unless, that is, these materials are combined (as in a composting system) with nitrogen rich ‘green’ materials. Using a combination of carbon rich mulches and nitrogen rich materials in layers is a form of composting in place, sometimes called ‘sheet mulching’.

Wood Chip/ Bark

Many gardeners not particularly familiar with no dig gardening techniques will think, when they hear the word mulch, or the bark or wood chips often placed around garden trees or shrubs.

Wood chip can come in different grades, or sizes. It can be made from a range of different types of wood. The type of wood used, the size of the pieces and several other factors will determine how useful if will be in your polytunnel garden. And it will dictate where it can and should be used.

Generally speaking, wood chip can be used as a mulch with impunity around trees, mature shrubs and other mature perennials. It can also be used on pathways. However, using wood chip as a mulch on growing areas used for annual fruits and vegetables is not quite as clean cut. And it may not be such as good idea.

Wood chips are low in nitrogen. This means the microorganisms that use the low-nitrogen organic matter as food (decaying it) must obtain the nitrogen they need from somewhere else. They take it, where no other nitrogen source is available, from the surrounding soil. Once they die, the microorganisms return the nitrogen to the soil. It is therefore made available to plants once more. But in the short term, there may not be enough nitrogen for nearby plants.

However, you could consider adding wood chip as a mulch, even around vegetables. To do so, consider adding a nitrogen-rich mulch material at the same time. That way, there will still be plenty of nitrogen to go round even while the decomposition is taking place. (We’ll discuss some nitrogen rich mulches later in this article.)

Straw

Another carbon rich mulch material is straw. Like wood chips, this material will be relatively slow to break down. And it can be great for retaining moisture and improving soil structure over time. But like wood chip (although to a somewhat lesser degree) it is lower in nitrogen and so can deplete soil nitrogen if a ‘green’ material is not also added.

Straw is often used as a mulch around strawberries and other fruiting plants to keep fruits up off the soil. It can also be used to aid in heat retention, and protect shallow roots from cold temperatures.

In sheet mulching, straw is used in combination with a nitrogen rich material to add plenty of organic matter as it breaks down.

General Purpose Organic Mulches

Some of the best organic mulches to use in your polytunnel garden are mulches that can protect the soil, but also add plenty of fertility for plants. There are a number of general purpose organic mulch options that will add nitrogen, phosphorus, potassium and other plant nutrients in relatively balanced amounts. These can be great choices for mulching around a wide range of edible crops throughout the year.

Compost Mulch

A good quality, home-made compost will always be a very useful thing to have on hand. The nutritional content of your compost will of course vary depending on what it contains. However, an average home-made compost will have 0.5% nitrogen, 0.27% phosphorus and 0.81 % potassium. It will also, of course, contain a range of micro-nutrients that will allow for healthy plant growth.

Compost should be spread around many common crops to allow them to grow strong. Getting into the habit of adding compost whenever a space opens up in your polytunnel, before you plant new crops into the area can be a very good thing.

Manure

Manures are also a good way to add fertility to growing areas. And some manures can also be used as mulch. Of course, we don’t all have access to it. But if you have a source of well-rotted manure from horses (especially good), goats, cattle or sheep, this can also be used as a mulch.

If you keep chickens or other poultry, their manure must be composted before use, as it can ‘burn’ your plants if added directly. Rabbits and other small rodents’ manure can also be added to your compost heap and then used as part of a mulch around your polytunnel crops. Worm castings (from a vermicomposting system) are also a form of valuable soil fertilizer that can be applied as a mulch.

Leaf Mold

Another idea that is great for mulches is leaf mold. You can make your own leaf mold by collecting autumn leaves and allowing these to rot down. Both partially decomposed leaf mold, and the finished product can be valuable as general purpose mulch for your polytunnel fruit and vegetable garden.

Spread leaf mold around the plants in your polytunnel in autumn to protect plants and soil and add fertility over the winter months, for example.

Nitrogen Rich Mulch

As mentioned above, certain mulches are high in nitrogen. These can help in breaking down carbon rich mulch materials when used in conjunction with them in sheet mulching/ composting in place. Nitrogen rich mulches can also be used alongside specific, nitrogen hungry plants to give them a boost and encourage healthy foliage growth.

Leaf Mulches (Chopped and Dropped Green Plant Material)

There are plenty of green, leafy materials that you can use as a mulch to add slow-release nitrogen fertilizer to your garden. Using organic green, nitrogen rich material is always a better option. In an organic, eco-friendly garden, you should always avoid adding any synthetic nitrogen fertilizers. So these mulches are one greener alternative to consider. Mulching with weeds (that have not yet gone to seed) is one good idea.

Grass Clippings

One of the best known nitrogen -rich mulches to consider adding to your garden is glass clippings. Often an abundant resource, grass clippings that have not been treated with any chemicals can make a great mulch. But it is important to remember that this and other nitrogen rich mulches should only be used around certain plants.

Too much nitrogen can sometimes burn crops, especially when the nitrogen rich material is not combined with carbon rich material. Too much nitrogen can also sometimes encourage excessive leafy growth, at the expense of flowers and fruits.

Nitrogen rich mulches are best used earlier in the growing season, when annual crops are just getting growing. They can encourage healthy root and leaf formation. But later, when certain crops are flowering and fruiting, it is best to use a potassium rich mulch instead.

Potassium Rich Mulch

A potassium rich mulch will promote healthy flowers and fruits. Most flowering and fruiting plants you might grow in your polytunnel will benefit from the addition of a potassium rich mulch around the time when they come into flower. Tomato feeds etc. are rich in potassium. But using your own potassium rich mulches (and making your own potassium-rich liquid fertilizers) can be a great alternative to buying some products.

Comfrey Mulch

Comfrey mulch is one common option. One of the reasons that comfrey is such a useful plant for organic gardeners to grow is that it is a dynamic accumulator. Its deep roots are fairly effective at drawing nutrients from far below the soil surface. And it is believed that when the comfrey is chopped and dropped, those nutrients collected are returned to the soil. They can then be taken up by other plants.

Though comfrey is not necessarily the best dynamic accumulator of potassium, it is relatively high in this important plant nutrient. And since it grows so quickly and abundantly, the plants can deliver a high quantity of leafy material to use as a mulch around other plants.

Other Plants That Effectively Accumulate Potassium (Dynamic Accumulator Mulches)

Comfrey is not the only plant that can make a good mulch material. Other dynamic accumulators that accumulate potassium effectively and might be used as a mulch include fat hen, pig weed, brassicas, purslane, and borage.

Mulch for Micro-Nutrients

Dynamic accumulators can also be chopped and dropped as mulches that will slowly release a number of other macro and micro-nutrients back into the soil for other crops.

Seaweed

One final mulch to mention is seaweed. Like so many other plants, seaweed can be a great source of the nutrients your polytunnel crops need. But seaweed can also provide a number of micro-nutrients that are not as readily available in other plants. This means that seaweed can be a great mulch. It can help add fertility to your growing areas. And it will also help to retain soil moisture and, when laid thickly, may also suppress the growth of weeds. If you live near a coastline, foraging a little seaweed to use as a mulch in your garden might be a good idea.

The mulches listed above are some of the most common and useful mulch options for your polytunnel garden. Do you have favourites? Which do you use in your polytunnel? And which crops do you use each one around? Share your tips, experiences and comments below.