Ask a Master Gardener: How to refresh perennial herb beds

Lift your perennials out in very early spring, prior to bud break.

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Lavender plants grow in one of the garden plots on the roof of the St. Louis County garage.
Clint Austin / 2014 file / Duluth News Tribune

Q: I moved into a house with a raised bed that has perennial herbs in it — lavender, sage and mint. The level of the soil is about 8 inches below the top of the bed. How can I raise it? Can I just add soil?

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A: Raised beds are a wonderful way to get plants to grow when the soil in your yard isn’t the best. My yard is clay and I would not be able to grow veggies that grow underground, like carrots, beets and potatoes, without my raised beds. Raised beds are also easier to keep weed-free. But the soil does tend to settle over time. That’s not a problem if you’re growing annuals; you can just add more. But perennials will need a different approach so that you don’t smother them or cause root rot.

I asked advice from other master gardeners and from Bob Olen, the St. Louis County horticulturist. The consensus was that you should lift your perennials out in very early spring, “prior to bud break,” Bob said. Add a mix of soil and compost to the bed until it’s at the level you want. Mix that with the old soil to about 12-15 inches deep and then replant.

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It’s a good idea to spread some mulch around your perennials. You can use shredded bark, wood chips, shredded leaves or straw. That will help keep weeds down and preserve moisture. Mulch also helps protect the roots from temperature swings. And it keeps your soil from blowing away.

One last suggestion: You might want to reconsider replanting that mint. Mint tends to spread aggressively and is hard to eradicate once it’s there. This moment when you’re adding a lot of new soil offers you a chance to get rid of it if that’s what you want to do.


It’s worth considering planting it in a pot rather than putting it directly back in the soil. You can even sink the pot into your perennial bed so it will look about the same but the plant will be confined. Use a pot at least 12 inches deep and make sure to water regularly. Mint likes moist conditions and will dry out faster in a pot.

Written by U of M Extension Master Gardeners in St. Louis County. Send questions to

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