Read "Making a Garden of Perennials" by W. C. Egan online for free on Textopian. Full text with search, annotations, highlights, and AI-powered reading aids.
Shallow-rooted plants like the cardinal flower ("Lobelia cardinalis") and the tall, fall-flowering hardy phloxes, dislike the hot sun beating down on their roots. Being surface rooters, and at the same time fond of moisture, they suffer when the surface soil is dried out. They should have a summer mulch to intercept the radiation of moisture from the soil.
The spent manure I mentioned as fine for covering bulbs, is splendid for this purpose and as it is of the same color as the soil, its presence is hardly noticeable; besides it adds humus. Almost any open material may be used, that will not offend our ideas of tidiness in appearance. Grass clippings from the lawn-mower may be used.