Before fungus attacks your fuchsia or whiteflies wilt your wisteria, raid the medicine chest. Dissolve three-quarters of an aspirin in a gallon of water, and spray indoor or outdoor plants with it every two to three weeks. Many plants produce salicylic acid (aspirin’s active ingredient) to fight disease; spraying them with it boosts this defense mechanism. Tomato and other plants treated with the solution in tests at the University of Rhode Island also yielded more fruit than those treated with commercial fertilizers.