Ideally, sweet woodruff should get 1 inch of water a week from rain or from a watering system, especially if it is not in a shaded woodland type setting where the soil is rich in organic matter which helps it hold moisture. Watch it closely during the hot, dry summer months to prevent it from drying out and dying back prematurely. Mulch new plants. (See below)
Sprinkle a general purpose granular fertilizer on the soil near sweet woodruff clumps in the spring. Take care not to let it touch plant foliage or stems. This will be sufficient for the season. A spray of dilute fertilizer or seaweed extract on woodruff foliage during the height of the summer may revitalize it somewhat. Avoid overfeeding, which stimulates foliage growth at the expense of the flowers.
Mulching helps control weeds, conserves soil moisture, and keeps soil from splashing up on the flowers. Cover the soil around new plants with a 1or 2 inch layer of chopped leaves, shredded bark, or other attractive organic material. Eventually the plants will blend together and cover the open soil between them. They will not need mulch then.
For more information see file on Using Mulch.
The easiest way to acquire more sweet woodruff plants is to divide existing clumps that have grown overlarge. This can be done in either the spring or fall. Carefully dig up the clumps and cut through the tangled, matted roots with a clean, sharp knife to make several smaller clumps. Replant these in the desired locations as described above. Sweet woodruff plants can also be started indoors from seeds. Follow instructions on the seed packet.