After the fifth year that I have been growing melons and having experienced both extremely hot and extremely wet and cold years, it is clear to me that fertilization, watering and cutting side shoots can have an influence on the yield, but the decisive factor that controls everything is and remains the temperature!
Every year since I have been growing melons, I have noted the weather data to better understand possible successes and failures. In 2022, I not only carried out a harvest survey of various varieties, but also tried to bring the melon season earlier in a staggered cultivation (without heating systems). The year 2022 was particularly suitable for this experiment because it was the warmest and sunniest year for Switzerland since measurements began! Accordingly, the data obtained is best suited for an interpretation of the possible future.
As explained in “When do melons actually bear fruit?”, planting in the foil tunnel usually takes place from mid-April, as the minimum temperatures at night usually no longer harm the plants. For 2022, however, 2 early plantings were carried out, one in mid-March (14.03.22), as well as a second early planting on (10.04.22) and direct sowing on 14.04.2022 and finally a regular planting (estimation based on night temperatures) on 20.04.2022. For spring 2022, the maximum and minimum outside temperatures are shown in the graph below. Despite the record year, temperatures still dropped to freezing point at the beginning of April 2022 and to just below 5 degrees Celsius from mid-April. The monthly average for April 2022 was around 10 degrees Celsius. Melons that are exposed to temperatures below ~15 degrees Celsius for several days stop growing completely. Temperatures below 10 degrees Celsius for several hours even lead to cold damage due to a lack of energy in the leaves, which means that not all metabolic products can be broken down. If the temperatures drop even further, or if these conditions persist, the plants will die within a few days.
The maximum temperatures from the end of March to the end of April were 25 degrees Celsius, the optimum growing conditions for melons. However, values of more than 15 degrees Celsius, when melons start to grow, were only achieved thanks to mostly sunny days. As soon as clouds appear in front of the sun, the maximum temperatures drop significantly, as the drop in temperature at the end of March and beginning of April during a few cloudy days shows. In theory, cultivation seems possible, provided the plants can be protected with a fleece or similar at night.
In unheated foil tunnels, a tropical climate is achieved on sunny days even with cold outside temperatures. But the low insulation values allow the heat to escape again quickly. Therefore, the factors “sunny days” and “minimum night temperatures”, as well as “isolation” are decisive for early cultivation.
Field Report
The first set of melons (planted after the 2nd true leaf), which was planted on 14.03.2022, was initially very weak due to very cold night temperatures, but the sunny days allowed the melons to survive the cold nights. However, growth was very limited. The second set of melons that were planted out on 10.04.22 (planting after the first true leaf) also had great difficulty in dealing with the low temperatures below 5 degrees Celsius at the beginning. By the time the “regular” set was planted on 20.04.2022 (3rd true leaf formed), all the melons that had been planted previously were about the same size (start of formation of 4th leaf), but of inferior quality to the new seedlings, and some had already withered/shriveled (12% of the plants). The early transplanted melon plants had deformed leaves that were smaller in size than the leaves of later transplanted melon plants. In addition, these were more frequently visited by pests and were therefore also greatly reduced in yield, or were disposed of during the vegetation season. The few plants that could be harvested were ready for harvest about 2 weeks earlier, with a few fruits.
Despite a record year in terms of temperatures, early planting in the unheated foil tunnel before mid-April was not recommended in 2022. However, the direct-sown melons and the regularly planted set show that it may no longer be necessary to raise melons for a good 4 weeks in future and that direct sowing can produce ripe fruit at the same time.
An early harvest season therefore makes little sense to me, especially if you want to remain environmentally friendly and do not rely on heating systems. Despite the warming climate in my growing region, I still get very cold and snowy April days, which destroy several weeks’ work in just a few hours. Therefore, planting from mid-April is still the earliest possible date in my opinion. Much more important for me is not extending the growing season by planting early, but by planting late. Spring is often still wet and cold, but the long-term meteorological data shows that fall is getting sunnier and drier in my region.
In 2017, I took a measurement of the outside temperature shortly after sunset almost every day (approx. 1 h). This was a simple way of obtaining an approximate mean value for the daily average temperature in my growing region. This graph also shows the limit of 15 degrees Celsius, which is the theoretical growth limit for melons. It is easy to see that a growing season for melons in 2017 is limited to the months from mid-May to around the end of August. This picture is accurate for all past years! Most melons can be sown until the beginning of June, as they need around 90 to 100 days to ripen the first fruits, which is around the beginning of September and thus the temperature drop below 15 degrees Celsius. As it is almost certain that in the coming years the average daily temperatures will not fall below 15 degrees Celsius until September, the last set could even be sown in July with special small-fruited varieties that need around 65 days to harvest!
So far, I have refrained from doing so due to lack of space and, above all, because the seed packets said “Sowing until June”. As soon as I have tried this on a larger scale, I will report on it here! But I am very confident that this will work very well in the future 🙂