With their big, round flowers, in soft, romantic shades, hydrangea bushes make a wonderful companion for both evergreen and flowering plants. Which plants you choose will, of course, depend upon what grows well in your particular area. If you tinker with the pH of the soil in order to change the color of your hydrangea flowers, that will need to be taken into account when choosing companion plants, as well. Although hydrangea is hardy to USDA Zone 8, temperatures below 30 degrees F may harm any buds on the plant.
The yellow-orange wild flowers of black-eyed Susan add a striking contrast to the soft, more formal blooms of the hydrangea shrub. The trick to growing the two together is that the hydrangea can't take quite the amount of sunshine that the Susans prefer. Planting the hydrangea next to the house, where it can get some relief from the afternoon sun, and the black-eyed Susan out front might be a solution.
Commonly known as monkshood, aconitum is a beautiful perennial flowering plant with purple flowers that grow on stalks. They love the same moist soil that hydrangea thrives in and do well in acidic soil. This is a very hardy plant that will take neglect and bloom all the way to late fall. Aconitum is extremely toxic and can be fatal if ingested.
Baptisia, also known as false indigo, is another plant that will look pretty planted with hydrangea. A low-growing plant with tall, flowering spikes, it blooms in yellow, white or blue. The flower stalks can grow to 12 inches in height and bloom in early spring.
If you are searching for more greenery to complement your hydrangea, inkberry is a plant to consider. A medium-sized perennial shrub, it is one of the few evergreen shrubs that will thrive in wet soil. The hydrangea would work well planted in front of it.