An asparagus fern's (Asparagus spp.) naturally green, ferny foliage typically turns yellow for two reasons: insufficient fertilizer, or if the plant isn't getting just the right amount of light. Lacy and delicate-looking, asparagus ferns grow outdoors in warm climates or as houseplants elsewhere.
The two species of asparagus fern you're likely to find at a garden center are foxtail or emerald fern (Asparagus densiflorus also Protasparagus densiflorus), which grows outdoors in U.S. Department of Agriculture plant hardiness zones 9 through 11, and lace or common asparagus fern (Asparagus setaceus also Protasparagus setaceus; formerly Asparagus plumosus), which grows in USDA zones 9 through 12. Both species can be grown as houseplants, and both species are susceptible to the same foliage-yellowing conditions.
Although asparagus ferns can tolerate some sun, full (all-day) sun -- or direct, afternoon sun -- causes the foliage to turn yellow. Dense shade also causes the leaves and stems to turn yellow.
Asparagus ferns do best in part shade. The amount of light the plants need for healthy growth is a balance between direct sun at one extreme and full shade at the other extreme. This means filtered sun or bright, indirect light for asparagus ferns. Move the asparagus ferns to a better area of the home or garden if they're not in part shade.
Asparagus ferns may produce flowers under the right conditions, but they are typically insignificant and these plants are primarily grown for their foliage. To keep the foliage from turning yellow, use a houseplant fertilizer -- even if your plants are outside.
Apply fertilizer once a week during the growing season, which is typically from early spring through September. If you're using a water-soluble fertilizer, use half of the recommended amount mixed in water, and apply it to plants according to the label directions. This is typically a rate of 1/4 teaspoon in 1 gallon of water for a water-soluble, all purpose 24-8-16 fertilizer.