While hibiscus won’t die in neutral or even slightly alkaline soil, they do prefer slightly acidic soils. The approach to providing this environment varies by the pH of your soil and water.
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Louisiana State University recommends a soil pH of 6.5 to 6.8 for hibiscus, whereas the University of Florida recommends a pH of 5.5 to 6.5, according to the University of Florida IFAS Extension. Suffice it to say that if your soil pH is between 5.5 and 6.8, you’re doing fine. Avoid alkaline soils because they retard nutrient uptake.
Amending for Acidity
First, test your soil pH. Avoid planting hibiscus near concrete, as it steers the pH to alkaline, the University of Florida reports. Amend your soil as needed with aluminum sulfate, elemental sulfur or coffee grounds.
Adjusting for Alkaline Water
If you have alkaline water, you may have to adjust the pH of your soil over time, since alkaline water will continually raise the pH of the soil. Test the soil after a year. If it’s higher than hibiscus like, consider top-dressing with coffee grounds a few times a year or adding acidifiers per your local extension service recommendations.