When driving past a potato field shortly before harvest, the sudden disappearance of the lush, green foliage is often noticeable. This is not a natural occurrence but a deliberate agricultural practice known as haulm killing or desiccation. Farmers intentionally destroy the above-ground green parts of the potato plant, called the haulm or vines. This calculated step in commercial potato production is executed with specific timing to manage the crop’s final development and achieve precise physiological changes in the tubers underground before they are harvested.
Stopping Tuber Growth and Achieving Uniform Size
The primary reason for haulm killing is to precisely control the final size of the potato tubers. As long as the green foliage remains healthy, the plant continues photosynthesis, converting sunlight into sugars transported down to the tubers, causing them to grow, or “bulk.” Farmers stop this energy flow by destroying the haulm, which immediately halts tuber growth.
Timing the desiccation is crucial because market specifications demand potatoes of a uniform size for different uses, such as salad potatoes or larger tubers for processing into fries. By monitoring the tubers underground, the grower can trigger haulm destruction the moment the crop has reached the most profitable size class. This artificial induction of maturity ensures a consistent size distribution across the entire field, maximizing the marketable yield. This control is especially important for seed potato production, where the goal is to produce smaller, uniform tubers suitable for planting the following season.
Toughening the Skin for Harvest and Storage
Halting the growth phase is also necessary to allow the potato skin to mature, a process called “skin set.” When the haulm is killed, the tuber stops expanding and begins a curing phase where the thin, easily damaged outer layer toughens into a durable periderm. This firm skin adheres tightly to the flesh and is less prone to scuffing and bruising during the mechanical harvesting process.
A well-set skin is a natural defense mechanism that minimizes moisture loss and resists the entry of decay-causing pathogens. Farmers usually allow a waiting period of approximately 10 to 21 days between the destruction of the haulm and the actual harvest for this skin maturation. Without this interval, the immature tubers would be easily skinned and damaged, leading to rapid spoilage and a drastically reduced storage life.
Reducing Disease Transmission to Tubers
Another significant purpose of haulm killing is to mitigate the risk of disease, particularly late blight, which is a destructive fungal-like pathogen. If the foliage is infected with late blight, spores can be washed down from the leaves and stems into the soil by rain or irrigation. These spores can then infect the tubers underground, leading to tuber rot that can spread rapidly during storage.
Removing the green foliage eliminates the source of active spores above ground, preventing this downward washing of inoculum onto the tubers. The desiccation process ensures the haulm is completely dead and dried out before the harvester passes through the field. This action creates a physical barrier, preventing contact between the harvested tubers and any potentially infected green tissue. Growers often wait a minimum of 14 days after haulm destruction in blighted crops to allow any viable spores in the soil to die off before digging up the tubers.
Techniques Used for Removing Potato Tops
Farmers employ a combination of mechanical and chemical approaches to achieve the necessary degree of desiccation. The most common mechanical method is flailing, which uses rotating chains or blades to physically shred and remove the majority of the green leaves and stems. Flailing provides immediate control of the canopy volume, which facilitates the subsequent harvesting machinery operation.
Chemical desiccation involves applying contact herbicides, historically including products like diquat, which rapidly killed the leaf tissue. However, due to regulatory changes, many of these rapid-acting chemicals have been withdrawn, prompting a shift toward alternative methods. Today, growers often rely on Protoporphyrinogen Oxidase (PPO) inhibitor herbicides, such as carfentrazone or pyraflufen, which are slower-acting and often require multiple applications to achieve complete stem kill.
The modern standard practice frequently combines these two methods: flailing first removes the bulk of the foliage, and then a chemical desiccant is applied to the remaining stem stumps. This two-step approach ensures a thorough kill of all green tissue, which is essential for maximizing skin set and disease control. Some newer, non-chemical methods like electrical weeding and haulm pulling are also being explored, especially in organic production, to address the need for effective top-killing.