Growing potatoes in a greenhouse allows for year-round cultivation, bypassing seasonal limitations to produce a consistent crop. This controlled environment is ideal for harvesting “new” potatoes outside the traditional season or ensuring a supply of specific varieties. By managing temperature, humidity, and light, a greenhouse provides optimal conditions that maximize tuber development. Container gardening within the structure further simplifies the process, making it an excellent method for a fresh, reliable harvest.
Selecting Varieties and Containers
Early or second early types, often called “new” potatoes, are the most suitable varieties for greenhouse cultivation due to their shorter growing cycle. Quick-maturing varieties like ‘Maris Bard,’ ‘Sharps Express,’ or ‘Red Norland’ minimize the window for potential disease issues in the confined space. These varieties are determinate, setting tubers at one level, which suits the restricted root space of containers. Indeterminate maincrop varieties produce tubers along multiple stem levels and require significantly deeper containers.
Choosing the right container is important for success. Fabric grow bags are popular because their permeable material promotes “air pruning,” encouraging a dense root system and providing superior drainage. Since grow bags dry out quickly, they require more frequent watering than solid-sided vessels. Any container must be at least 15 inches deep to allow for adequate hilling and tuber development, and it must feature sufficient drainage holes to prevent waterlogging.
Preparing the Greenhouse Environment and Seed Potatoes
Maintaining a stable environment reduces plant stress and encourages steady growth. Potatoes thrive when the daytime temperature is between 64 and 75 degrees Fahrenheit, and night temperatures should not drop below 45 degrees Fahrenheit. An optimal relative humidity level for most greenhouse crops hovers around 80 percent, which supports plant physiological processes.
If cultivating during winter or early spring, supplemental lighting is necessary to ensure adequate light for photosynthesis. During intense summer heat, shading the greenhouse glass may be required to prevent scorching and overheating, which inhibits tuber formation. Before planting, seed potatoes should be “chitted” by encouraging short, sturdy sprouts to form. Place the seed potatoes in a cool, light, frost-free area for four to six weeks to develop these sprouts, giving them a head start once planted.
Planting and Initial Hilling
The planting medium should be light, loose, and well-draining to accommodate expanding tubers and prevent rot. A mix of quality potting soil and compost is an excellent choice, providing initial nutrients while keeping the medium aerated. Potatoes prefer a slightly acidic soil with a pH between 5.2 and 6.4 for optimal health.
To plant, fill the container with 4 to 6 inches of the growing medium. Place the chitted seed potatoes sprout-side-up onto this layer, ensuring each piece has at least one eye. Cover the seed potatoes with 3 to 4 inches of soil, completing the first stage of hilling. This initial covering is essential because tubers turn green and become inedible if exposed to light.
Managing Ongoing Growth and Troubleshooting
The confined volume of container soil requires a more attentive watering and fertilization schedule than in-ground gardening. Consistent moisture is necessary, but careful attention must be paid to avoid overwatering, which can lead to tuber rot. Potatoes are heavy feeders and have a particularly high demand for potassium during the tuber bulking phase. A fertilizer formulation that is high in potassium, such as a 14-7-21 blend, should be applied as the plants grow, especially once the foliage is established.
Continued hilling is a defining practice in container potato growing. This must be performed as the stems grow taller, typically when the foliage reaches 6 to 8 inches above the current soil line. Hilling involves adding more soil or compost around the stem until only the top few inches of leaves are visible. This action stimulates the plant to produce more tubers along the buried stem.
The closed environment of a greenhouse can create conditions that favor specific pests and diseases, necessitating proactive management. High humidity combined with poor airflow increases the risk of fungal issues like late blight and powdery mildew. Manage these by ensuring excellent ventilation through fans or strategically opening vents. Common indoor pests, including aphids and whiteflies, should be monitored closely. Address infestations immediately with insecticidal soap or biological controls before populations become established.
Knowing When and How to Harvest
Timing the harvest depends primarily on the type of potato being grown and the desired size. For ‘new’ potatoes, which are prized for their thin skin and delicate flavor, harvesting can begin approximately two weeks after the plants start to flower, typically 60 to 70 days after planting. At this stage, the plant’s foliage is still green, and a few tubers can be carefully “robbed” from the container without uprooting the entire plant.
To harvest maincrop potatoes intended for storage, the foliage must be allowed to yellow and die back completely. This signals that the plant has directed its energy into maturing the tubers. Once the foliage has died, leave the tubers in the container for an additional two weeks to allow the skins to thicken and cure, which improves their storage life. The simplest method for retrieving the crop is to gently tip the entire contents onto a tarp or wheelbarrow, minimizing the risk of damage.