Following planting, sprouts develop from eyes on seed tubers and emerge from the soil. The roots begin to develop at the base of emerging sprouts. During this stage the seed tuber is the only energy source for growth. Adequate phosphorus for early growth and root development, and phosphorus starters are often applied at planting.
Leaves and branch stems develop from above-ground nodes along emerged sprouts. Roots and stolons develop at below-ground nodes. Photosynthesis begins. Stage I and II typically last 30 to 70 days depending on environmental factors. The latter part of vegetative growth as growth accelerates and through tuber bulking is the period of greatest nitrogen demand, but excess nitrogen can delay tuber initiation and thereby shorten the tuber bulking stage.
Tubers form at stolon tips but are not yet appreciably enlarging. This stage lasts about two weeks. In most cultivars the end of this stage coincides with early flowering. Adequate nutrients are needed during this time to promote abundant tuber initiation. Reduced tuber initiation will reduce yields at the end of the season.
Tuber cells expand with the accumulation of water, nutrients, and carbohydrates. Tubers become the dominant site for deposition of carbohydrates and mobile inorganic nutrients. Tuber Bulking is the longest stage and usually lasts 45-60 days. This is a period of rapid nutrient uptake. During this time a constant supply of nitrogen is needed, and potassium uptake can reach as much as 14 lbs. K2O per acre per day.
Vines turn yellow and begin to lose leaves. Photosynthesis decreases, tuber growth slows, and vines die. Tuber dry matter reaches a maximum, and tuber skins set. When growing a long season variety (i.e. Russet Burbank) in an area with a short growing season, the plant is usually killed using a herbicide to allow the tuber to grow larger before harvest. Plants should be running out of nitrogen by this time. Excess nitrogen can prolong maturation, delay skin set, and reduce quality.