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Using Stones and Shells for Mulch

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Q: I am re-landscaping my entire yard, and instead of mulch, rocks or pine needles, I am planning on using shells. Will shells create any type of problem for my trees, shrubs or flowers?
(Question from Naples, Florida)

A: Generally shells are much like a stone mulch. Except shells will slowly leach calcium into the soil as they very gradually degrade. If you already have a high pH (soil measure of acidity or alkalinity), which means your soil is alkaline, then using a shell mulch would slowly raise the pH. Also most Florida well water has a high (alkaline) pH which will also raise the pH of the soil over time. And generally most plants in our landscape prefer or do better with soils that are just slightly acidic. So you should have your soil pH tested at your local Cooperative Extension office. It is usually a county department listing. Also, shells, like stones, used for mulch will be warmer than an organic mulch layer. And plants generally do better when soil temperatures are moderate not hot.

Having said that I still would encourage you to use organic material for the mulch for the following reasons:

Our Florida soils are sandy with little organic matter and organic matter is beneficial for plant growth by aiding water retention and slowly breaking down to provide nutrients to the plants. Organic matter also provides higher levels of micro-organisms and humus that are beneficial to soils and plants. Once you decide to stop adding organic mulch to your soil, the existing organic matter in the soil decomposes within several months and is gone. And there is no additional organic matter other than an occasional decaying root that will replenish this valuable soil asset — organic matter.

Organic mulch is by far the best thing and one of the least expensive things you can do for the long-term health of your landscape. And the organic mulch should be placed directly onto the soil with no weed mat which would inhibit the breakdown and movement of the organic mulch into the soil.

As you can see I do have a organic mulch bias. But I have developed this bias over the years watching poor sandy soils become poorer and plants doing poorly.

Good luck however you decide to proceed.

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