United States public firms bought 11.2% of the green patents they obtained between 1996 and 2021. I show that firms buy green patents when examination delay leaves them with knowledge but no right to exclude others while a product-market rival files in the same technology class. Instrumenting delay with examiner slowness, I find that a one-standard-deviation increase in delay raises the probability of a purchase in the following twelve months by 11.7 percentage points, against a base rate of 16.3%. Firms without such a rival show no significant response, and accelerated examination under the Green Technology Pilot Program reduced purchases. The patents bought have claims closer to the firm's pending claims than other patents traded in the same technology subclass and year, which indicates a demand for legal protection rather than for knowledge. The response is concentrated in green technology, whose inventions combine more technological fields. After green purchases, funds oriented toward green innovation are less likely to exit the buyer, and emissions from purchased energy are lower when the patents cover production processes.