Although mulch won’t attract ants, ants often build their nests in it. Mulch helps keep the ground moist, which ants like. Since ants are by nature decomposers, they like the decaying wood and will help break it down. Ants can be a nuisance when you’re trying to garden, however, so finding a mulch that will repel them seems like an eco-friendly solution. While some types of mulch have proven to deter ants, they won’t keep ants out of your garden or home entirely.
Aromatic Cedar Mulch
Aromatic Eastern red cedar (Juniperus virginiana) mulch discourages ants from nesting in the mulch. That does not mean, however, red cedar mulch will keep ants away. According to a 2003 study published in the “Journal of Economic Entomology,” ants would willingly cross red cedar mulch to find food, but fewer ants were found on trees surrounded by the mulch. No studies have been done on the effectiveness of Australian red cedar (Toona ciliata) or Western red cedar (Thuja plicata) or on “true” cedar (Cedrus spp.) mulch.
Cypress and Other Wood Mulch
Cypress seems to have some repellent qualities, but is not as consistent as aromatic red cedar mulch. According to a 2003 study published in the “Journal of Economic Entomology,” cypress mulch was as effective as cedar mulch around flower beds, but ineffective when placed around oak trees or alongside buildings. Other wood mulches have shown little effect at repelling ants. In fact, tramp ants like Argentine (Linepithema humile) and odorous house ants (Tapinoma sessile) frequently nest in pine straw, pine bark and other hardwood mulches.
According to its advertisements, rubber mulch will not attract ants or termites. No scientific studies have been published that either support or dispute this claim, but even if rubber mulch keeps out ants, it may have negative effects on the environment. Curtis Swift and Linda Chalker-Scott, extension specialists with Colorado State University and Washington State University respectively, have found that rubber mulch is ineffective at keeping out weeds, adds chemicals to the soil that are toxic to plant growth, and is highly flammable and difficult to extinguish once it begins burning.
Mulch Plus Chemicals
Research studies are being done to test the effectiveness of combining chemicals with mulch at repelling ants, according to the 2010 Imported Fire Ant and Invasive Ant Conference. Botanical extracts from mint, sage, cypress, hardwood bark, oak, juniper and pine have shown some effectiveness at repelling red imported fire ants. Organic chemicals known as terpenoids from American beautyberry, Japanese beautyberry and cedar are also promising. Combining DEET with mulch may also work at repelling ants. As of 2011, none of these have been commercially released.