I have a solution, but it requires some knowledge of science.
QUESTION 1: When we lose weight, where does the weight go? In the loo? Nope! We breathe most of it out from the lungs. The body burns the food and this releases carbon dioxide. That's why you have to breathe faster when you exercise, because you need to breathe in more oxygen to burn and breathe out more to get rid of the carbon dioxide.
QUESTION 2: Where does the stuff come from to make trees? From the ground? Nope! The tree needs the ground for water, a few nutrients, and mostly anchorage. The wood, leaves and fruit comes from the carbon dioxide in the air. That's right: trees are made from air.
And grass works the same way. The stuff inside grass comes from the air, and the bigger the grass plan or weed, the faster it can convert carbon dioxide into plant stuff.
So the solution is obvious: (1) don't let the plants get too big and (2) deprive them of carbon dioxide from the air. You can either surround them with sealed lid that holds in a vaccuum, or get your wife to go outside and personally throttle each one until it dies.
I hope that helps.