Can I grow watermelons in small polytunnels?
Mar 10, 2026
As a supplier of small polytunnels, I often get asked if it's possible to grow watermelons in these compact structures. The short answer is yes, you can indeed grow watermelons in small polytunnels. In this blog post, I'll delve into the details of growing watermelons in small polytunnels, including the benefits, challenges, and practical tips to ensure a successful harvest.
Benefits of Growing Watermelons in Small Polytunnels
One of the primary advantages of using a small polytunnel for watermelon cultivation is the ability to extend the growing season. Watermelons are warm-season crops that thrive in temperatures between 70°F and 85°F (21°C - 29°C). By using a polytunnel, you can create a microclimate that protects the plants from cold temperatures, frost, and adverse weather conditions. This allows you to start planting earlier in the spring and continue growing later into the fall, potentially increasing your overall yield.
Yes - you can absolutely grow watermelons in small polytunnels.
With proper ventilation, vertical support, and the right variety, even compact tunnels can produce sweet, healthy fruit.
Another benefit is the protection from pests and diseases. Small polytunnels act as a physical barrier, preventing insects and other pests from reaching your watermelon plants. This reduces the need for chemical pesticides, making your crop more environmentally friendly and healthier to consume. Additionally, the controlled environment inside the polytunnel can help prevent the spread of fungal diseases, which are common in humid conditions.
Small polytunnels also provide better control over the growing environment. You can regulate the temperature, humidity, and ventilation to create the ideal conditions for watermelon growth. This is particularly important during the germination and flowering stages, when the plants are most sensitive to environmental changes. By maintaining optimal conditions, you can ensure that your watermelons grow strong and healthy.
There are other benefits of growing in small tunnel greenhouses.
Improved Heat Retention for Faster Growth
Small polytunnels trap solar warmth during the day and release it slowly overnight. This temperature stability is extremely valuable for watermelon plants, which require warm soil and air temperatures to grow vigorously.
In cooler climates, outdoor soil temperatures may remain too low for optimal watermelon development. However, inside a polytunnel, the environment can be 5–10°F warmer, which significantly accelerates germination, vine growth, and fruit formation.
Protection From Heavy Rain and Wind
Excess rainfall can damage watermelon plants by compacting soil, spreading fungal diseases, and washing away nutrients. A polytunnel acts as a protective barrier that keeps rain off foliage and fruit.
According to the Royal Horticultural Society, protected growing environments can significantly improve fruit production for heat-loving crops.
Wind protection is another major advantage. Strong winds can break vines or disturb pollination. Inside a tunnel structure, vines remain stable and protected.
Challenges of Growing Watermelons in Small Polytunnels
While there are many benefits to growing watermelons in small polytunnels, there are also some challenges that you need to be aware of. One of the main challenges is space limitation. Watermelon plants are vigorous growers that require a lot of space to spread out. In a small polytunnel, you may need to train the vines to grow vertically or use trellises to maximize the available space. This can be a bit more labor-intensive, but it's a necessary step to ensure that the plants have enough room to grow.
Another challenge is pollination. Watermelons are pollinated by bees and other insects, and in a closed environment like a polytunnel, there may not be enough natural pollinators. To overcome this, you can hand-pollinate the flowers using a small brush or cotton swab. Simply transfer the pollen from the male flowers to the female flowers, and you should see fruit start to develop.
Choosing compact watermelon varieties is the biggest success factor.
Bush or mini varieties like Sugar Baby and Mini Love perform far better in small protected spaces.
Finally, maintaining the right temperature and humidity levels can be a challenge in a small polytunnel. If the temperature gets too high, the plants may wilt or suffer from heat stress. On the other hand, if the humidity is too low, the plants may dry out and become more susceptible to pests and diseases. You'll need to monitor the environment closely and make adjustments as needed to ensure that the conditions are optimal for watermelon growth.
Practical Tips for Growing Watermelons in Small Polytunnels
Now that you're aware of the benefits and challenges of growing watermelons in small polytunnels, let's take a look at some practical tips to help you succeed.
Choose the Right Variety
Not all watermelon varieties are suitable for growing in small polytunnels. Look for compact or bush varieties that are specifically bred for container or small-space gardening. These varieties typically have shorter vines and smaller fruits, making them a better fit for the limited space inside a polytunnel. Some popular varieties include Sugar Baby, Golden Midget, and Mini Love.
Prepare the Soil
Watermelons require well-draining soil that is rich in organic matter. Before planting, amend the soil with compost or well-rotted manure to improve its fertility and structure. You can also add a slow-release fertilizer to provide the plants with the nutrients they need throughout the growing season.
Plant at the Right Time
Watermelons are sensitive to cold temperatures, so it's important to wait until the soil has warmed up before planting. In most regions, this means waiting until after the last frost date. You can start the seeds indoors a few weeks before the last frost and then transplant the seedlings into the polytunnel once the weather has warmed up.
Provide Support
As mentioned earlier, watermelon plants need support to grow vertically in a small polytunnel. You can use trellises, stakes, or cages to train the vines to grow upwards. This not only saves space but also helps to improve air circulation around the plants, reducing the risk of fungal diseases.
However, providing support is not the only thing to be considered. Garden hobbyists have to take into account the limited space in their backyard greenhouse, and other parameters.
Managing Vine Length in Limited Space
Watermelon vines can grow 10–20 feet long, which can quickly overwhelm a small growing structure.
Gardeners can manage this by:
- pruning secondary vines
- guiding vines along trellises
- selecting compact watermelon varieties
This helps maintain airflow and prevents overcrowding inside the polytunnel.
Preventing Overheating in Summer
While heat is beneficial, excessive temperatures can stress watermelon plants.
During peak summer months, polytunnels may exceed 95°F (35°C). To avoid heat stress:
- open side vents
- roll up tunnel sides
- add shade cloth if necessary
Maintaining airflow is critical for healthy plant growth.
Vertical growing is the secret weapon for small polytunnel gardening.
Trellising vines dramatically increases productivity while saving valuable floor space.
Water and Fertilize Regularly
Watermelons are heavy feeders and require regular watering and fertilizing to grow properly. Water the plants deeply once or twice a week, depending on the weather conditions. You can also apply a balanced fertilizer every few weeks to provide the plants with the nutrients they need.
Monitor the Environment
As I mentioned earlier, maintaining the right temperature and humidity levels is crucial for watermelon growth. Use a thermometer and hygrometer to monitor the environment inside the polytunnel and make adjustments as needed. You can open the vents or doors to increase ventilation and reduce the temperature, or use a misting system to increase the humidity.
Soil Temperature Matters More Than Air Temperature
Many gardeners focus only on air temperature, but watermelon roots are highly sensitive to cold soil.
Ideally soil temperature should remain above 70°F (21°C) for healthy root development. Inside a polytunnel, raised beds or black mulch can help warm soil faster in early spring.
Hand Pollination Improves Fruit Set
Inside small polytunnels, natural pollinators such as bees may be limited. Hand pollination ensures reliable fruit development.
Steps for hand pollination:
Identify male flowers (thin stem)
Identify female flowers (small fruit behind bloom)
Use a small brush or cotton swab
Transfer pollen between flowers
This simple process dramatically increases fruit success rates.
Also, I've found a watermelon growing guide for you, happy reading.
Our Small Polytunnel Products
At our company, we offer a range of small polytunnel kits that are perfect for growing watermelons and other crops. Our Small Polytunnel for Home Use Kit (10ft Polytunnel) is a great option for beginners or those with limited space. It's easy to assemble and provides a protected environment for your plants.
If you need a larger polytunnel, we also offer the Large Polytunnel Professionnal Kit (20ft Wide). This kit is ideal for commercial growers or those who want to grow a larger quantity of watermelons. It's made from high-quality materials and provides excellent protection from the elements.
For those who need a more compact option, we have the Polytunnel Greenhosue 2x2x2m. This small polytunnel is perfect for growing a few watermelon plants in a small backyard or balcony.
Before we conclude this article, we'd like to share a few tips about growing these fruits in hoop greenhouses.
Conclusion
Growing watermelons in small polytunnels is a great way to extend the growing season, protect your plants from pests and diseases, and have more control over the growing environment. While there are some challenges to overcome, with the right variety, soil preparation, support, and care, you can enjoy a bountiful harvest of delicious watermelons.
If you're interested in purchasing a small polytunnel for growing watermelons or other crops, please don't hesitate to contact us. We'd be happy to help you choose the right kit for your needs and provide you with any support or advice you may need.