Daylilies are mainstays in Midwestern gardens but they flourish in a variety of settings from USDA zone 2 to 9, including seaside plantings. The State of Hawaii lists them as a "Zone 2" groundcover, meaning that they are moderately salt-tolerant. None of this changes the fact that daylilies, like other perennials can suffer saline damage, either from overdosing on ocean beaches or, more likely, from winter street salts.
Sodium and chlorine in salt water, air and de-icing products block the plant's intake of potassium and magnesium, both necessary for active photosynthesis. Limited photosynthesis curtails chlorophyll production, weakens the plant and causes its long narrow leaves to curl and shrivel. Less chlorophyll restricts the plant's food production for flowering, resulting in smaller buds that are slow to open. Weaker plants are more susceptible to infestation by insects like fungus gnats and fungal daylily rust disease.
Plant daylilies in sandy loam or other well-drained soils to avoid salt damage; moderately salt-tolerant plants will blanch and die if their roots sit in salt just as will plants with no tolerance. Heavy clay soils hold water in the topsoil for long periods of time, "burning" roots and drying the daylily's rhizomes that store its food. Sandy loam is best for daylilies that will have to deal with salt air, street salts or inaccurate fertilizer spreader calibrations. At least 2 inches of compost and manure or humus and sand should be worked into heavy soils with regular exposure to salt.
Plant seaside daylilies on the leeward side of a building or behind a sheltering dune to protect their foliage from desiccation by salt-laden ocean winds. Set daylilies in from streets, sidewalks and alleyways to separate them from mechanically-spread salt. Limit the use of fertilizers around daylilies; they don't need extra nitrogen--or the sodium present in many fertilizers. Give them a 1-inch summer mulch of clean compost; remove the old compost and replace it in the fall to provide all the nitrogen they need. It will make weed-pulling easier and absorb spillover lawn fertilizer or de-icing compounds.
Water plants deeply when they are "burned" by street salts or too much nitrogen fertilizer. Once the salt is in the soil, the best way to help the daylily is to force salt down through the soil away from plant roots. Extra water leeches salt through the topsoil into the subsoil. Give daylilies a 1-inch summer mulch of clean compost; remove the old compost and replace it in the fall. It will provide all the nitrogen they need, make weeds easier to pull and protect their "feet" from spillover lawn fertilizer or de-icing compounds.
Watch for signs of saline injury in spring; plants that are discolored or have smaller, less numerous flower buds may indicate salt damage. Spring saline injuries may signal a need for extra potassium; use top dressings of organic sources of potassium like greensand, manure or hardwood ashes. Compost mulch and green kelp contain both potassium and magnesium. Unless you can identify saline injury as the sole culprit for this damage, be sure to eliminate other possibilities like overcrowding when diagnosing general weakness in growth and bloom. Acidic or sandy soil may make it difficult for daylilies to use magnesium, also resulting in yellowish leaves. If you have questions about soil salinity, you should contact your state university's agricultural extension for a low-cost soil test.