Parsley is a classic planting companion to rose bushes. Historically, the pairing has been thought to dissuade beetles from colonizing around roses, increase the vigorousness of rose growth, and increase the fragrance that develops in the rose blooms themselves. The bushiness of the green parsley can also camouflage the bare lower canes of rose plants, making a rose bed look more polished. Since parsley is relatively shallow rooted it will not interfere with the rose roots and can be under-planted by sowing seed or using mature herb plants.
Loosen the top few inches of soil surrounding your rose plant or plants with a cultivating fork or hand trowel to create a good medium for the parsley roots to easily establish themselves.
Sow parsley seeds in the spring or early summer, starting 6 inches out from the main rose trunk and extending as far as the edge of the planting bed or to where you want the parsley to end. Nestle the seeds into the top inch of soil and cover lightly with the soil to hold them in position. Water in well until the soil is saturated with a gentle stream so as not to shift the seeds in the soil.
Dig small planting holes around your rose plants that are a few inches larger than the root mass of your mature parsley plants. Place holes no closer than every 6 inches to allow the plants to spread out and start at least 6 inches out from the main rose canes.
Slide the plants from their nursery containers and place in the holes so that the top of the root mass is level with the surrounding soil. Back fill the displaced soil around the parsley plants to secure them in position and tamp down the soil lightly to collapse any air pockets. Water in well until the surrounding soil is drenched but there is not standing water. Maintain at least lightly moist soil around both plants at all times.
Prune down parsley plants in the late fall after the first hard frost comes and the plants are wilting. In warmer climes where parsley will grow year-round, prune away only dead plant material to keep the parsley looking tidy.
Maintain your established rose fertilizer regimen around the base of the roses and do not add any additional fertilizer for the parsley. As an herb parsley is not a heavy feeder and will benefit from the rose fertilizer nutrients in the soil.