Great groundcovers help conserve soil moisture and leave little room for weeds to grow. This is true of many colorful, fragrant sages (Salvia spp.) that spread freely.
For example, Meadow Sages such as Adora Blue (Salvia x superba 'Adora Blue')not only offer flower spikes profuse with blossoms but also grow in mat-forming clumps that discourage weeds. Cornell University recommends Adora Blue's close relative Blue Hill Sage (Salvia nemorosa 'Blauhugel') as a weed suppressant due, in part, to this clumping characteristic.
However, it may be that the essential oils creating the pleasant aromas of many Salvias are also helpful in suppressing weeds. Many researchers refer to this apparent trait as the “Salvia phenomenon.”
Italian Study of Five Sages
A 2010 report about a study from Italy’s University of Salerno shows that the essential oils of five Salvia species are antigerminative, which means that they deter other seeds in their surrounding soil from germinating. To read the report, which appeared in the journal Molecules, click here. Then, when the page opens, click on “Full Text PDF” in its left margin.
The study focused on chemicals called terpines in the following species:
The researchers applied the essential oils of the plants to the seeds of two salad foods -- radish (Raphanus sativus) and garden cress (Lepidium sativum), then observed their germination and initial growth. They discovered that, except for Golden Sage, the oils of all of the Salvias deterred growth.
Salvia leucophylla Research
Salvia has long been a subject of studies on allelopathy, which is the ability of one plant to inhibit the growth of another. In the 1960s, researchers concluded that thickets of California Purple Sage (Salvia leucophylla) -- also known as California Gray Sage -- and White Sage (Salvia apiana) were surrounded by bare soil due to their terpenoids warding off other plants.
However, a 2012 report on a Japanese study about S. leucophylla, which appeared in the journal Current Bioactive Compounds, disputes the assertion that plant chemicals are solely responsible for this bare zone. For example, the Japanese report concluded that additional factors impact surrounding plant growth, including the grazing of small animals and seasonal changes. Also, the Japanese report said it's unclear exactly how other plants absorb the terpenoids of S. leucophylla.
Sages aren't a magic bullet for avoiding weeds in your landscape, but they offer an edge. Upcoming Salvia Small Talk posts will discuss groupings of groundcover and border sages from USDA Cold Hardiness Zones 5 to 11 in order to help you create an attractive garden with little room for weeds. Meanwhile, please contact us if you have any questions. We're always glad to help and share what we know.