For flower beds that are close to downspouts using a landscape rock around the water outlet is the best option. You do not need to use rocks throughout your garden bed but using them under the area with the greatest water flow will help to keep your soil in place and protect your plants. Garden edging like pavers, steel edging or plastic edging will hold any lighter mulches like bark or tree mulch in place and stop if from being washed out of your flower bed. Flowers will have sensitive roots so it is important that the roots are protected from mulch and soil run off. Smaller rocks can work to hold the soil in place for plants that are very close to your downspout.