Asparagus ferns are evergreen perennials that thrive in a range of conditions. They can handle drought, heat, poor soils and overwatering, as well as light conditions from full shade to full sun. Leaf yellowing, however, is a symptom of stress in your asparagus ferns.
Asparagus ferns are well-adapted to many conditions -- full shade, partial shade and full sun. But when exposed to full sun, or to afternoon and evening sun during the summer, the leaves begin to yellow. Though not lethal, this is a signal of stress on the asparagus ferns, and they will grow better if given more shade.
Lack of nitrogen or micronutrients such as manganese, sulphur and iron cause leaf yellowing in most plants, including asparagus ferns. If you've fertilised your asparagus ferns with nitrogen, chances are the deficiency is a micronutrient. Compost contains a wide profile of nutrients, and will correct most deficiencies.
Plant asparagus ferns in partial shade to prevent yellowing in the summer sun. Add finished compost to your potting soil or to your garden bed to improve the nutrition profile as a whole. Apply nitrogen-rich fertilisers, such as manures or fertilisers with a high N number on the N-P-K label, in the spring or early fall.