Nasturtium leaves turn yellow when there is a lack of adequate direct sunlight and damp, well-drained soil. Nasturtiums thrive when they're in direct sunlight for at least half a day with a combination of hot or warm days and cool nights.
Surface crust forms on soil after each rain or watering, and cultivating it helps to revive stunted nasturtiums. Applying a mulch of well-rotted compost by gradually turning it into the soil also helps revive nasturtiums. Side dressing the plant in the spring and again in the summer with a specialized, watered fertilizer protects and nourishes nasturtiums. Without ample sunlight and a constant supply of moisture at the roots, nasturtiums can't thrive. Young nasturtiums will not become productive adult plants if the leaves wilt prematurely and are consistently dried out. Cutting them for bouquets or to eat helps them to bloom. It also encourages lateral development which results in more flowers, but only if any yellowing mature leaves are promptly removed.
Aphids are pests that are notorious for their attraction to nasturtiums, especially the shoots as they are so tender. Pyrethrum is a spray effective in getting rid of them, but insecticidal soap should not be used because it burns nasturtium leaves. If not sprayed with a poisonous pesticide, all parts of the nasturtium are edible. The leaves and shoots add a peppery mustard flavor to a mixed salad. The blossoms contain a sac of nectar that is both sweet and sour. The seeds can be pickled as capers when they are nearly mature but still green.