Before you fill your raised bed you need to decide if you want to add plastic sheeting to the inside. We at GreenGuys Raised Beds have learned through trial and error that plastic can be very helpful to increase the longevity of your wooden raised bed. Plastic sheeting has several benefits to your raised beds. The first benefit is to prevent weeds from coming up through the side and bottom. Plastic sheeting is also a requirement if you use pressure treated wood for your raised bed. The heavy duty plastic sheeting helps prevent the leaching of chemicals into your garden soil from the treatment applied to the lumber. Another benefit of plastic sheeting is it will also create a barrier from the wet soil against the wood. Ants and termites love a raised bed and will happily munch on your wooden raised bed and create their own habitat that you possibly won’t see till they have eaten halfway through the wood. Therefore, plastic lining the inside of your new raised bed can be very beneficial.
After you decide on adding plastic then you can start filling your raised bed. Depending on the height of your bed, you might not want to add the whole bed full of soil. Despite it being costly to fill a raised bed with soil it’s not necessary. Make sure your bed has at least 6 to 12 inches of good soil for the roots to have plenty of room to grow. There are many items you can use to fill the bottom of your bed. Here is a list of the most common items we have seen added to the bottom of the raised bed before adding soil.
Filler for Raised Beds:
sticks, twigs, or wood stumps(organic matter)
gravel or rocks for drainage
grass clippings and/or straw
combination of sandy soil, vermiculite, organic compost, peat moss, potting soil or good garden soil
compost (you can use homemade compost, worm castings, and more)
animal manure
cardboard
It is all about the layering when adding filler for your raised bed. This is something you can do to help fill those garden boxes, without filling them entirely with nutrient rich soil and compost. There is a bonus to this, because the items you use in the bottom will eventually turn into mulched compost that will feed your plants for years.
We like to add cardboard to the bottom of the raised bed as a first step. It can act as a weed barrier and will decompose nicely. Then you can add large limbs, wood, tree branches on top of the cardboard. You can clean up the yard by placing your twigs and limbs and these will also fill up some space in your raised bed. Also by placing larger items in the bottom it helps fill up some cubic yards with less expensive options which are still beneficial to your raised bed.
Next you can add some larger gravel/stones to fill in around the wood. You could also add pea gravel or rocks you have around the yard. Be careful to not add too much rock because it’s best to add materials that will decompose. This will create some nice drainage in your raised bed garden. Drainage is important and these rocks will help keep your raised bed from just holding water. You do not want your soil to be waterlogged. Most raised beds dry out quicker than in ground beds but having a raised bed hold water won’t make a successful garden. Finding the right combination of materials is easy to help you have a bountiful garden.
Next we added a layer of grass clippings straight from our backyard. We made sure that none of it had been sprayed with weed killer or fertilizer so that it would be as organic as possible. Over time these grass clippings will break down and help create some amazing garden soil. The soil level will depend on how much space you want for your garden plants roots. I would recommend filling it to the top, because over a short amount of time your soil will settle. If you want to plant root crops, make sure your soil will be deep enough to accommodate them. We wanted about 12 inches of good soil. Eventually the entire garden bed will be rich soil as the materials at the bottom break down, but that takes time.
After the grass you can add a layer of straw or leaves. This filled about 1/2 of the garden beds. So already, we only have to fill the garden beds with 1/2 the amount of dirt, and over time, the wood, grass clippings and straw will turn into a great compost. You can add more or less depending on what you have available.
After these are laid down,you can add a layer of sandy soil. This will serve a couple of purposes. First, it will help to fill in any holes or voids in the rocks/straw/wood combination. Second, it will weigh down the material underneath adding a bit of pressure which will help with the process of decomposition. And last, it will mix with the organic compost, vermiculite and other garden soil and different materials to create a well balanced soil.
The last thing we did was add a thick layer of organic compost, organic mulch, animal manure, dry leaves and organic garden bed soil. This created a nice loose soil and also great soil quality. The excess water will drain through the gravel and wood at the base of the garden beds. But remember that each raised bed can be filled however you choose. Sometimes after we add straw and leaves combination we add bags of garden soil. There is not a perfect way to fill a raised bed and you will have success whichever way you choose. We have also gotten a great deal on a truckload of soil and just filled the raised bed up after a few pieces of cardboard and wooden logs.
As time passes you will notice that the level of soil will settle and lower, This is just your bed settling. You will need to continue to add more compost and organic matter as we see the need. Some feel the best time to add it is in the fall so that it can have the winter season to break down and do its thing. We will add a layer of leaves in the raised bed in the winter if we are not using that bed for winter crops. But normally we will add new soil in the spring when we start planting seeds/seedlings.
With the layering practice, you will experience some settling in the raised garden beds, especially after you begin watering, but this is easily fixed. You can gradually add more soil, or wait until the end of your growing season and you can add more then. If you plan to plant root vegetables such as carrots or potatoes, make sure your soil is deep enough. You can gradually add soil on top of your potatoes as they grow. Another option is coconut coir. It is a great option to add to your garden beds as a base layer before your good soil.
Raised garden beds can be a very rewarding way of gardening. We have been using raised beds for many years now and I steadily request several new beds a year because they are just so much easier than plowing and tilling gardens. You need to pull weeds in a raised bed, It’s a snap. The initial cost of the raised bed is easily worth the years of use you will get from raised beds.