Mode of Action
Glyphosate works as a systemic herbicide that works its way from the plants' foliage and stems into the meristem tissue. The chemical effectively inhibits a particular enzyme in plants by disrupting the shikimic acid pathway, which is specific only to plants and certain microorganisms. Inhibiting the enzyme results in stunted growth, leaf malformation and death of tissue; some plants may also lose their green coloring. Plants may take anywhere from a few days to a few weeks to die completely.
In the 2010 graduate thesis for Auburn University, "Roundup Over the Top of Container-Grown Nursery Crops," which was approved by various university professors of horticulture and agronomy, several nursery crops were treated with Roundup, a widely available glyphosate-based herbicide. Two juniper selections, "Blue Rug" (Juniperus horizontalis "Wiltonii"), thriving in U.S. Department of Agriculture plant hardiness zones 3 through 9, and "Blue Pacific" (Juniperus rigida conferta "Blue Pacific"), hardy in USDA zones 5 through 9, were included in the study. Although "Blue Pacific" was injured at the 1/2 pound application rate in spring, it recovered quickly. "Blue Rug" was tolerant of rates up to 2 pounds during the February application, but showed stunted growth in June and September.
Weeds in Junipers
Thriving in USDA zones 3 through 9 depending on species, many juniper selections are low-growing shrubs that form dense carpets, ideal for ground cover, ledges or slopes. Unfortunately, however, weeds and grasses can make their way through the prickly branches and become eyesores. To control these weeds with glyphosate, wear long sleeves and protective gloves, and apply spot treatments of glyphosate -- many products have foam sprays or narrow-stream sprays for such treatments. You can also "paint" the leaves of the weeds with glyphosate, taking care not to overly apply the product to your juniper. However, because junipers can handle fairly large doses of glyphosate without permanent damage, an accidental spray to your prized ornamental shouldn't do very much damage.
Selective herbicides are those that affect a limited number of plants, unlike glyphosate, which has the potential to damage all plants. Pre-emergent herbicides will prevent weeds from coming up around your juniper; post-emergent herbicides control already-sprouted weeds. Beware: junipers are sensitive to dicamba, a selective herbicide labelled for many woody plants and broadleaf weeds. Certain products are labelled for use on specific junipers, but not others, and may cause damage to those not listed.