(News article for August 2, 2025)

My last article, Troubleshooting Raised Bed Problems: Part 1, discussed excessively high nutrient levels in raised bed gardens and how to submit a raised bed substrate sample to the LSU Soil Testing and Plant Analysis Lab.

Another raised bed issue that I’ve run into is excessive drainage. Good drainage is one reason we use raised beds, but if they’re too well-drained, it can be hard to keep plants adequately watered.

Some potting mixes, by themselves, drain too well for raised beds. Because of their structures, pots need substrates that provide really good drainage, and one should use a true potting mix when planting in pots. In raised beds, though, some potting mixes will need to be mixed with other ingredients to provide adequate water-holding capacity.

Likewise, much of what’s sold locally as topsoil is made up largely of aged pine bark. This is a well-drained material. That’s helpful when it’s used to improve drainage and add organic matter to soil, but it needs to be mixed with other ingredients if used in a raised bed. Aged pine bark has low nutrient levels, so it can be mixed with high-nutrient organic materials, such as those mentioned in the last article.

While the last article focused on the issue of having nutrient levels that are too high, excessively low nutrient levels are problematic, too. Beds filled with low-nutrient materials will need to be supplemented with other organic materials, synthetic fertilizers, or a combination of both.

A final issue I’ve seen in raised beds is injury from herbicide residues. Tomatoes with distorted growth are the most common complaint, but other vegetables can also be affected. This sometimes happens when people have used manure from cattle or horses that have eaten pasture grass or hay that’s been treated with one of several herbicides that are very persistent. It can also happen when people use hay treated with one of these materials as mulch. Some herbicides that have the potential for significant carryover include ones with the active ingredients picloram, clopyralid, aminopyralid, and aminocyclopyrachlor.

Note that the potting mix samples mentioned in the last article are checked for plant nutrients, pH, and soluble salts – the report will not tell you if you have herbicide residues. You can contact us if you have concerns about this.

Let me know if you have questions.

Contact Mary Helen Ferguson.

It can be hard to keep plants in a raised bed adequately watered when the substrate is made up largely of materials like aged pine bark and sand, which tend to drain quickly. (Photo by M.H. Ferguson)

This tomato plant has symptoms of injury from an herbicide in the synthetic auxin group. (Photo by M.H. Ferguson)