QUESTION: Should I put rocks in the bottom of my raised garden bed? My neighbor told me that it’s good for drainage. – Lauren O.
ANSWER: This information from your neighbor is not accurate, unfortunately. You should avoid putting rocks or gravel at the bottom of your raised garden beds, or any of your planters or containers for that matter. One of the longest running gardening myths is that lining the bottom of your planting containers or garden beds with rocks will improve the drainage capabilities of the potting soil that you use, which is quite untrue.
Lining the bottom of your planting spaces with rocks is a pretty big mistake for several reasons. Putting rocks at the bottom of your raised beds and containers actually increases the water saturation level by creating an artificial water table on the bottom layer of your planting space. With your buried rock layer trapping water beneath your soil, problems with fungal growths and root rot are more likely to occur. What’s more, the gravel will eventually begin to mix in with your potting soil, making it impossible to get the rocks out as you turn your old soil while mixing in new compost and topsoil between growing seasons.
The rocks may provide additional drainage at first, making it seem like they are helping at the bottom of your raised beds. However, as time goes by, the topsoil will loosen and fall into the rock layer, plugging up the spaces between the rocks and slowing down drainage significantly. Then, not only will you have a slow draining medium, but you will also have to remove all of the rocks from your soil so that you can keep using the soil in the beds. Once the rocks and the soil have merged together, removing all the rocks, especially if they are gravel or pebble sized rocks, will be a very tough task to manage.
Instead of rocks or gravel, make your bottom layer out of organic materials that will eventually break down into the soil and enrich it in the process. What constitutes organic materials? Well, lots of stuff. You could line the bottom of your raised beds with unfinished compost, allowing it to finish breaking down underneath layers of soil. You could also use leaves, old dry wood, like twigs and small branches, or even lawn trimmings.
Keep in mind that wood will take longer to break down and enrich the soil than leaves, grass clippings, or unfinished compost would. You may notice the topsoil level in your beds beginning to sink as the organic material on the bottom breaks down and incorporates itself into the soil. This is a perfectly natural process, and should cause you no concern. You will just need to add a little more topsoil the next time you prepare your beds for future plantings.
There are many different options of things to use instead of rocks or gravel to lighten your soil and increase the drainage in your garden beds. Organic materials offer the best options, as they increase drainage while they are breaking down, and enrich the soil once they break down, and never will need to be removed from the beds at a later date. Any property with trees on it should provide an excess of unwanted leaves each fall, and what better use to put those leaves to than as the bottom layer of organic material in your raised beds?
Lining the bottom of your raised beds with rocks can also be an insufficient deterrent for weeds, as weeds can easily grow up in the spaces between rocks. This can be avoided by laying down layers of landscaping fabric or burlap between the rock layers and the soil layers, which would also keep much of the soil from falling in between the rocks as well. But with so many better options of organic materials to line the bottom of your beds with, which also breaks down to enrich the soil above over time, there are much better choices available to anyone building their raised beds.
Rocks have been used for ages in container gardens when using planters with no drainage holes, or planters that could easily blow over in the wind and need extra weight to keep them from tipping. However, in raised beds, there is no need to add rocks to the mix, as it could end up being more trouble than it is worth later on down the line when you need to remove all those rocks.
Instead of using all of your rocks or gravel on the bottom of your raised beds, put it to use instead, as a lawn free walkway between and around your raised beds. Having a gravel walkway in the working areas around your raised beds can have several helpful functions. First of all, you don’t have to mow the areas between your beds, which can be awkward and cramped to do anyway. Plus, with a gravel walkway, there is no chance of any weeds growing up around your raised beds, which should help deter weeds from growing up in the beds themselves as well.