Choose a planting site for the Matthiola incana flowers that receives full sun to light shade and consists of moist, fertile, well-draining soil. Prepare the area by tilling to a depth of at least 6 inches with a garden tiller and incorporating a 2- to 3-inch layer of organic compost into the soil prior to planting.
Plant Matthiola incana seeds in the garden after the danger of frost has passed and temperatures reach 50 to 55 degrees Fahrenheit. Sow the seeds no more than 1/2-inch deep, as they need light to germinate, and space them 12 to 18 inches apart to allow room for the full mature size of the plants.
Water your Matthiola incana flowers about once a week, or frequently enough to maintain consistently moist soil. Do not allow the soil to dry completely or wilting may occur. In addition, do not allow standing water to accumulate, as the flowers are prone to developing root rot.
Apply a 1- to 2-inch layer of organic mulch to the soil surrounding your Matthiola incana flowers to promote the soil's moisture conservation and protect the roots against the summer heat. Leaves, grass clippings, bark chips and straw are all suitable mulches.
Feed your Matthiola incana flowers with an all-purpose garden fertilizer just after planting to stimulate new growth and help the plants establish themselves in the garden. Follow the fertilizer manufacturer's instructions on the label for proper dosage and application.