The Q&A Archives: Raised Bed Over Gravel Base

Question: I have an existing 40 x 40' area covered with about 1 foot of small gravel (it used to hold solar panels), and I would like to put raised beds there. The bed is flat, in full sun, on top of a small mountain with strong wind.

In order to avoid shoveling out the gravel, I am planning to put a layer of landscaping fabric on top of the gravel and then put a 1-foot deep layer of soil and organic material on top of that. I am planning to grow just about everything - perennials, annuals, and vegetables. I thought that the gravel would continue to keep the weeds at bay and provide good drainage.

After more thought, I am not sure this is a good plan. Will the 1 foot of soil be inadequate for good root development? With the strong sun and winds on top of a mountain, do I need to worry about drainage? Would I be better off to shovel the gravel? How deep does a raised bed need to be?

Answer: In my experience, this type of raised bed can be quite successful, although it may be so well drained that it requires some care in watering. This will depend to some extent on the top layer and its water holding capacity as well as the amount of mulch you use and the choice of plants. Your best approach is to add as much organic matter as possible to the soil mix. Organic matter improves the soil structure so it can hold both air and water, among other benefits.

While it is possible that some of the deeper rooted perennials will not be happy there, most annuals and vegetables should be fine. I am not sure why you are planning on using the landscape fabric, since it is unlikely that any weeds would come up through a foot of planting medium topped by mulch. You may find that some of the organic matter shifts down through the gravel, but as this happens you will also find that earthworms and other beneficial living things will be able to move into the beds. This intermingling will also help with any deep-rooted plants. (I am envisioning here a foot of gravel or less.)

Wind will certainly also be an important factor--but you would have a problem with that regardless of what type of soil you are using. Wind can have a significant effect on winter hardiness, as well as dry things out, so you might want to look into a windbreak in any case.

Finally, drainage is important in several ways. Most plants do best with an evenly moist yet well drained soil (meaning not soggy). While many plants will tolerate some extra moisture in the summer, those sensitive to drainage will resent that excess in winter and show their discontent by dying. I suspect the gravel bed will provide plenty of drainage. If you are planning on growing perennials the best approach is to experiment at first and see what does well for you over time, but that is true in any perennial garden.

My main concern with your project is the possibility that someone used long term herbicides on the gravel, or perhaps layered plastic beneath it in an effort to preclude weed growth around the panels.

Good luck with your project!

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