Your best choice is moving your perennials to another shady area in your yard. Planting new trees is another option, but because your garden is large, it will likely take your new trees too long to grow enough to produce the shade your plants need. Erecting a pergola or trellis with vines to create shade over your garden is another option, but the cost may be prohibitive. You might also consider selling some of your plants and using the money to purchase plants that will like the newly sunny spot.