Hostas are a shade perennial appropriate for use in non-tropical gardens and regions. It is a deciduous plant, losing its foliage in winter and resprouting each spring. Leaf yellowing can be caused by many factors, including too much sunlight, lack of soil moisture, infertile soils, soggy soil and the onset of winter dormancy.
Too Much Sunlight
In general, the hosta looks and grows best in partial to full shade light exposures, such as under the dappled to dense shade of trees. When sunlight is most intense, in the middle of summer, too much exposure can cause sunscald or leaf burn, the yellowing and browning of foliage. Over-exposure to hot sunlight causes leaf temperatures to increase and requires additional soil moisture to offset this imbalance. Usually the leaf dies rather than the hosta plant over-compensating and expending energy in maintaining the leaf in such an inhospitable environment.
Through decades of hybridization and plant selection, hosta plant breeders have found some varieties that tolerate considerable direct sunlight exposures. These plants are an exception rather than the rule with hosta, however. In cool summer climates, full sun exposures on hosta can yield beneficial and presentable results. In regions with dry soil, arid air and hot summer temperatures, increased exposure to direct sunlight normally causes leaves to scald, yellow and die.
If the hosta plant is exposed to dry soil or an exposed drought, the natural response will be to expend foliage in order to retain more water in crucial plant tissues, mainly the roots. Dry soils can cause the hosta to enter an early dormancy in an effort to weather the adverse conditions and hopefully re-sprout next spring when moisture is again plentiful.
If the soil is nutrient poor the hosta will have difficulty sustaining any plant growth. Sandy soils that lack any organic matter are particularly likely to cause scrawny or unattractive hosta plant growth. Moreover, soils that have been poisoned, such as being drenched with a petroleum product or herbicide, will also retard hosta growth.
Lack of nutrients, specifically nitrogen, magnesium and manganese, is known to cause healthy leaves to become a sickly yellow color, too.
Hosta roots are not tolerant of soggy, waterlogged soils. Although they may withstand a rare, brief inundation of rainfall, standing water on the roots and stems of hosta leads to suffocation of roots and fungal rot problems.
An attack by insects or other pests can cause leaf yellowing. Insects that suck juices from foliage, such as aphids, can deplete leaves so they become yellow before dying. Snails and slugs are also known to relish hosta foliage, and although the rounded holes in leaves is indicative of their presence, they also may chew at leaf stems, causing the entire leaf to yellow.
Onset of Winter Dormancy
No hostas are evergreen. The are naturally programmed in their genes to become dormant in autumn when temperatures are cold and day lengths are short. Once autumnal frosts invade the garden, the foliage will yellow and die back to the ground. People in regions with mild, warm winters, such as in the subtropics, find that hostas are not long-lived, since they never experience the cold weather in winter to force the necessary winter dormancy.