Clean up the vegetable garden. Now's the time to clear out old and languishing vegetables from your warm-season garden. If you leave summer squash, tomatoes, cucumbers and melons in your garden too long after they cease to be productive, they can develop a powdery mildew, which can spread to other plants. Be proactive and clean out old edibles to get ready for cool-season greens, squashes and herbs.More: Regional garden checklists