A new invasive weed that can grow into a dense mat, choking off most other plants, has gained its first North American foothold in Carlsbad and is threatening to spread across the Southwest with seeds that can travel on clothes, boots and tires. Ward’s weed, a native of the Mediterranean region and Western Asia, has already conquered Australia. It arrived on the small continent in 1915 and now can be found in every Australian state, with the most extensive concentrations on the country’s arid southern coast.