I am planning to install some new walks and a patio at my home and have heard that I should not use limestone screenings for the base. Is that correct? If so, why should this readily available product be avoided?
— Cindy Thompson, Morton Grove
Although limestone screenings — a fine gravel — make a stable base for pavers, they are very alkaline and will increase the pH of adjacent soil. This can be harmful for some plants.
Most soils in the Chicago area already are alkaline, with a pH above 7. This is hard on plants such as river birch and red maple that are sensitive to alkaline soils, and having limestone in the soil nearby would make the situation worse.
Soil that is too alkaline interferes with some plants’ ability to absorb nutrients, causing them to develop chlorosis, or yellowed leaves. Chlorosis is an indication that the plant cannot make sufficient chlorophyll.
Talk to your contractor about alternatives to using limestone fines to set pavers. For example, grade 9 gravel (road construction-grade gravel) that is used with fines (gravel with particles ranging between 1 and 2 millimeters) will compact well and form a good base under brick pavers or flagstone. Considering its use in road construction, it’s no surprise that grade 9 gravel is also excellent for use under driveways and walkways. It creates a solid sub-base that can take considerable weight.
Many patios and walks also are installed on a gravel-and-sand base.
Tim Johnson is director of horticulture for the Chicago Botanic Garden in Glencoe
