What is garlic mustard (Alliaria petiolata)? This cool-season biennial’s leaves and stems emit the smell of garlic or onion when crushed. Plants are 12 to 48 inches in height, and in their second year, produce numerous white flowers with four separate petals. Garlic mustard is the only plant of this height in the woods with white flowers in May. Hand pulling before flowering is recommended. It is believed to have been brought to North America by European settlers for use in cooking and medicine.
Here is the link to a brochure created by the Oregon Department of Agriculture: http://www.oregon.gov/ODA/shared/Documents/Publications/Weeds/GarlicMustardBrochure.pdf
How does it spread? This weed spreads exclusively by seed. The plant exudes a toxin from its roots into the surrounding soil and kills off competing seeds (the allopathic substance actually prevents germination of any seeds except its own!). It also stunts the growth of nearby plants. English Ivy in all its evil glory can’t hold a candle to this marauder. It’s clearing the way so this plant can take the next area over.
Why it’s bad, very bad: The concern surrounding garlic mustard comes from its ability to aggressively invade a woodland community and displace the native plant community to include grasses, shrubs, perennials, and tree seedlings.
How to remove it: Mowing is not an effective control because plants will still bolt and seed. Mowing spreads garlic mustard seed like wildfire – do not mow when seed pods are present (May through September). Hand pulling the weed is easiest during early bolt (2nd year). Difficult during rosette stage (first year) except for small patches. Multiple years are needed to exhaust seed bank. Pull at base to avoid breaking stem. All pulled plants should be bagged and removed from site (seed will set and/or plant will re-root).
If you must use chemicals, use a product that contains glyphosate or Tricloypr. To avoid damaging native forbs, spray the rosette stage during late winter/early spring. If not sure how to identify rosette stage, you can spray during flowering. Fall application to the rosettes (after some rain evens so plants are growing again) may also be effective. Sprays at height of summer will not do much. Use aquatic formulations when spraying near any body of water. A combination of chemical and hand pulling is very effective – pulling bolted plants and spraying right after pulling.
I am not suggesting growing it for cooking, but while eradicating it from your property, you could get even with it by making a very tasty pesto from the leaves. Here’s just one recipe I found on line: http://www.mnn.com/food/recipes/photos/6-edible-invasive-species-recipes/garlic-mustard-pesto