It's best to avoid these problematic plants.Credit: Katrin Ray Shumakov / Getty ImagesPerennials are a great way to spruce up your garden and add splashes of greenery to your landscape. However, not all of them are worth planting. Some are very difficult to grow, are invasive, or can have negative effects on neighboring plants and flowers.So how do you know which perennials are worth growing—and which you should avoid altogether? We spoke with g…