perennials in raised beds

16 years ago

I've got my perennial herb garden in a raised bed that was filled with a mixture of topsoil, peat moss, and bagged composted manure.

The bed is up against a concrete retaining wall, and the front is made of those stackable concrete wall stones with the lip on the back. I ended up raising the entire bed by one row of stones this year, which meant that I needed to move every last plant. Ouch.

So. This year I got quite a bit more shrinkage in the bed than I got last year. I clearly need to raise the soil level either this Fall or in the Spring, but I fear that just adding too much of something (topsoil, compost, what-have-you) around the existing plants may not be good for them.

I may end up purchasing more topsoil for some of the topping up these beds, because the ratio of organic matter in them is awfully high, leading to lots of shrinkage. I'd like to get to a point where annual shrinkage is at a manageable level.

How do those of you who grow perennials in raised beds deal with this problem?

Thanks!

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Comments (3)

kqcrna
16 years ago
I'm not sure you can get around this. Only thing I know to do is to dig up the plants and add soil.

Adding organic material is great for the soil. But over time that OM is consumed by the soil food web and the level shrinks. It's a constant cycle, but until you add actual mineral soil, I don't think you can get the level to stay higher. And even that soil will start to disappear over time by erosion, wind, loss of soil when yanking or transplanting plants...

Karen
gardengal48 (PNW Z8/9)
16 years ago
To add to the level of the bed you will need to dig and lift the plants first -- very few plants will tolerate much additional soil or whatever piled over their roots or next to their crown. In fact, planting too deeply - which is what this translates to - is a great way to do them in rapidly :-)

And I'd agree to omit adding any more organic matter. First, it will just extend the problem and second, a great many perennial herbs prefer very lean, fast draining soil with minimal organic matter. Too rich a soil tends to encourage flowering, which draws flavor from the foliage and lessens its intensity.
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leira
Original Author
16 years ago
Karen, I'm glad to hear that you also have the inclination that adding mineral soil is the right answer. When creating the beds, I wanted to add good soil with lots of organic matter, and I also wanted to avoid spending a fortune on it. The topsoil/peat/manure mix was good for that, and it's been an absolutely fabulous growing medium with excellent water-holding capacity but also excellent drainage, but overall was probably too high in the organic matter for long-term stability.

It's also possible that the heavy rains earlier this year contributed to the higher rates of decomposition and therefore more shrinkage.

I will say that I'm not looking forward to digging up and replanting all of those plants, plus all of the additions to the bed, yet again. The herbs are even bigger, being a year older, and even last year they had larger root systems than I ever would have imagined. Some are due for dividing, which is fine, but leaving the others exactly where they are would have suited me.

Maybe I can get by another year with the current levels, and only moderate topping-up.