Potatoes are a staple food for many people around the world. They are versatile, nutritious, and easy to grow. When growing potatoes, a common question that arises is whether potatoes need complete darkness to grow properly or if some light exposure is acceptable.
The potato plant
The potato plant (Solanum tuberosum) is a member of the Solanaceae or nightshade family. Other plants in this family include tomatoes, eggplants, and peppers. Potatoes grow best in cool climates with moderate rainfall and soil that drains well. They originate from the Andean mountain region in South America.
The potato plant produces tubers, which are modified underground stems that enlarge and develop “eyes” or buds. The eyes sprout and produce new potato plants when planted. Tubers develop at the end of underground stems called stolons. Stolons branch off from the lower portion of the stem right above the seed potato that was originally planted.
Above ground, the potato plant grows leaves, flowers, fruits, and stems like any other plant. The foliage produces food through photosynthesis. This food provides energy for leaf and stem growth above ground and tuber formation below ground.
Potato tuber development
There are four main stages of potato tuber formation:
- Sprouting – the seed potato begins to sprout stems and leaves
- Vegetative growth – above ground foliage expands and stolons begin to form underground
- Tuber initiation – stolons start swelling at their tips to form small tubers
- Tuber bulking – tubers rapidly expand in size and accumulate starch
After the potato plant flowers, tuber growth slows down or stops completely. The plant begins focusing energy on seed production rather than vegetative growth. Maximum tuber size is reached shortly after the plant finishes flowering.
Tuber initiation and bulking occur during the underground portions of the potato plant lifecycle. This is the phase where potatoes grow rapidly in size.
Do potatoes need darkness to form tubers?
Early potato research demonstrated that potatoes form tubers most readily when underground portions of the plants are kept in complete darkness. Any light exposure suppressed tuber formation.
One reason is that light causes the potato plant to synthesize chlorophyll and undergo photosynthesis. When the underground stolons are exposed to light, they turn green and actively produce foliage. The energy is directed toward above ground growth rather than developing tubers.
Darkness signals to the plant that it is time to start enlarging tubers underground since above ground expansion is not favorable. Keeping the underground portions in complete darkness maximizes the plant’s energy directed toward rapid tuber bulking.
Early research on potatoes and darkness
In the 1920s, American botanist William Stuart published a series of experiments examining the impacts of light exposure on potato yields. Some key findings included:
- Potatoes exposed to a dark period of at least 14 hours per day for 3-5 weeks after planting formed significantly more tubers than potatoes exposed to shorter dark periods.
- Continuous light inhibited tuber formation completely. The largest tubers developed when plants had complete darkness underground.
- Brief 5-10 minute exposures to light did not impact tuber formation as severely as longer exposures.
- Light during early tuber initiation was more detrimental to yield than light exposure after tubers began developing.
Stuart concluded that potatoes require long uninterrupted dark periods of around 14-16 hours to form tubers most effectively. Disrupting this critical stage of development with light severely impacts yields.
Modern potato production techniques
Today, commercial potato production follows many of the same principles established by early research about potato plant physiology and light exposure. Some common practices include:
- Hilling – drawing soil up around the base of potato plants. This prevents light from reaching the underground portions.
- Night planting – planting seed potatoes in the evening or at night to provide plants with long uninterrupted dark periods.
- Vine killing – mechanically killing above ground growth about 2 weeks before harvest. This eliminates a source of light exposure to the tubers.
However, modern high-density potato production utilizes shorter dark periods, often around 12 hours.rather than the 14-16 hours suggested by early research. Shorter dark periods can lead to higher total yields in some scenarios.
How much light can potatoes tolerate?
Research over the past 50 years has shown that potatoes can tolerate some light exposure during tuber formation without significant impacts on yield:
- Most potatoes form tubers effectively with a 12 hour photoperiod (light exposure).
- Light intensities below 10-15 lux (units of illumination) do not seem to inhibit tuber formation.
- Greening of tubers (synthesis of chlorophyll) starts at light intensities of 30-50 lux or higher.
- Tuber growth declines slightly once light intensity exceeds 10-15 lux, but plants can still form tubers at higher light levels.
Based on this research, complete darkness may not be essential for potato tuber development. However, yields tend to be highest when underground portions are kept in darkness as much as possible.
Impacts of light exposure
What happens if developing tubers do get exposed to light?
- Less overall tuber formation and lower total yields.
- Smaller tuber size and/or uneven tuber growth.
- Greening of tubers due to chlorophyll synthesis, which can lead to accumulation of bitter and potentially toxic glycoalkaloids.
- Premature termination of tuber growth if plants start focusing resources on foliage production.
While potatoes can tolerate some light, excess illumination during tuber bulking has clear detrimental effects on yield and tuber quality.
Using artificial light to control tuber formation
Since the potato tuber development process is so tightly linked to photoperiod (light and dark periods), scientists have experimented with using artificial light to manipulate tuber growth.
By interrupting the dark period with light, tuber formation can be delayed. This technique can be used to stagger harvests for a continuous potato supply. It can also prevent tubers from forming during warmer summer months when conditions are unfavorable and lead to growth of poor-quality tubers.
For example, one study utilized night interruption lighting from 10 pm to 2 am to inhibit tuberization during July and August. This allowed high-quality tubers to be produced in September and October instead.
Artificial lighting in storage facilities also helps suppress tuber sprouting after harvest. Exposing tubers to continuous light maintains dormancy and prevents premature sprouting during storage and transport.
Growing potatoes in containers
Growing potatoes in containers has become a popular gardening trend in recent years. This method allows potatoes to be grown in limited space. However, containers pose some unique challenges for managing light exposure.
Some tips for growing potatoes in containers include:
- Use containers at least 12 inches (30 cm) deep and preferably opaque or darkened containers to block light.
- Cover container edges with soil, mulch, or burlap to prevent side lighting.
- Move container to area with less light once plants reach 6 inches (15 cm) tall to induce tuber formation.
- Provide plants with 12 hours or more of uninterrupted darkness for best yields.
Growing in containers near a light source like a window risks green tubers and poor yields. But with proper management to minimize light exposure, healthy potato plants can thrive in containers.
Hydroponic potato production
Hydroponics is a method of growing plants without soil using only water and mineral nutrient solutions. Since the plant roots are exposed rather than underground, preventing light exposure is a major hurdle for hydroponic potato production.
Some strategies to produce potatoes hydroponically include:
- Covering nutrient solution tanks or channels with opaque lids or plastic sheeting to keep out light.
- Using black containers to block light from reaching roots.
- Suspending shade cloth above the setup to reduce light intensities.
- Applying light-blocking spray-on latex coatings to the developing tubers.
With sufficient light exclusion using these methods, potatoes can be grown hydroponically. However, yields are often lower and tubers are more prone to greening compared to soil-grown potatoes.
Conclusion
Early research showed potatoes require complete darkness to form tubers effectively. However, we now know potatoes can tolerate some light exposure during underground growth, although complete darkness is still best. Light during tuber initiation and bulking inhibits yields and can reduce tuber quality.
Common potato growing methods are designed to block light from reaching underground tubers. But with proper management, potatoes can be successfully grown in containers, hydroponically, and in other settings with potential light exposure.
While not essential, providing potatoes with complete darkness during tuber enlargement minimizes negative impacts of light and allows the plants to channel energy into maximal tuber growth. So for the highest yielding, best quality potato crop, ensure developing tubers are kept in the dark as much as possible!