By Anna Mariscal, Parham Gardener
My first growing season at Parham in the summer of 2020 – with temperatures regularly above 30°C, dry weather, and blazing heat off the Garden’s stone walls and pale Fittleworth paths – convinced me that melons can and should be grown at Parham. This may seem naïve, especially now, given the weather we have had during the 2021 growing season. However, I am an optimist and a born and raised Californian of the opinion that a summer without melons is hardly a summer at all.
Truthfully, an attempt at growing melons in pots as cordons in a glasshouse was already taking place at Parham during the 2020 summer. The plants quickly became stressed, likely due to extreme temperature spikes and dryness in the glasshouse, the restrictive size of the pots, and a growing medium lacking rich organic matter. As the season progressed the melon plants became infested with whitefly. Given other pressing demands in the garden and the degree to which the melon plants were infested, the plants were eventually thrown into the compost.
With my experience of growing melons in California, I started to consider what techniques would result in successfully cultivating melons at Parham. It seemed that if we were going to have any success, we would need to 1) provide ample space for roots, 2) provide good air circulation, and 3) provide a growing medium rich in organic matter. However, as we moved into the autumn and winter seasons of 2020, I shelved away my thoughts on melon growing.
In January 2021, I attended a No Dig lecture by Charles Dowding, hosted by the Kew Mutual Improvement Society. Mr Dowding shared information regarding a hotbed, constructed by layering manure and straw, to build heat in his glasshouse and provide warmth for seed germination. Mr. Dowding explained that by late spring he relocates the hotbed to the outdoors and transplants heat-loving fruiting-plants (such as aubergine) directly into it. My mind turned back to melon growing and wondered if we could try growing melons at Parham in such a way – there would be ample room for roots, warmth, and rich nutrients.
Soon after attending Mr. Dowding’s lecture, I came across Monty Don’s series The Secret History of the British Garden. In the first episode, he visited Hampton Court Palace, where one of the palace’s period kitchen gardens had been restored, featuring melon hotbeds. In the words of Monty Don himself, “Melons would have been grown in hotbeds, which were an important feature of any aspirational garden in the 1600s.” Further explained by the Gardener, Vicki Cooke, a fresh straw and manure mix from the stables would be used to construct the hotbeds, and as the mixture breaks down heat is generated creating the warm environment needed for growing melons.
After hearing that 17th century gardeners were growing melons, it seemed only more in our favour to try growing melons in this manner, especially with a warming climate. Having presented the idea to Parham’s Head Gardener, Andrew Humphris, and receiving his approval, it came down to figuring out how to bring this idea to fruition. Surprisingly, there is not much written information available regarding constructing or utilizing manure hotbeds for melon cultivation.
In fact, I could only find one reference to manure hotbed construction from an old gardening book I was given. Published in 1980, Getting the Most from Your Garden: Using Advanced Intensive Gardening Techniques by Dan Wallace, provided me with the following insight: “The French gardeners spent a long time perfecting their manure-composting system to get the right amount of bottom heat at the right time. Most of the specifics…were never written down and have been lost, but further experimentation with manure-heated hotbeds could revive this art.” As electricity and glasshouses became cheaper to produce, heating techniques using manure became less fashionable. Seventeenth century gardeners developed the technique using the means and local resources they had at the time; it seems to me that these sustainable methods are indeed coming back into fashion today.
While I could not obtain an exact ratio of horse manure to straw used by gardeners 400 years ago, Getting the Most from Your Garden provided me with one more piece of helpful information:
“Uneven heating is the bane of dung-composting hotbeds, and since the manure is buried beneath growing plants, there is no way to replenish the manure once it has stopped heating. Decomposing fresh manure can be expected to give off heat for 6 to 12 weeks; if it is mixed with kitchen waste or contains a lot of straw, the daily fluctuation will be less severe and the heating period will be lengthened.”
Once we at least had an idea of what the methodology was behind manure hotbed construction, our second and third hurdles in their construction were where to construct the hotbeds and where to source the manure and straw. Initially we considered growing the melons within one of the parterres in the vegetable garden but, ultimately, we decided the already constructed south-facing cold frames within the garden nursery were the ideal location. Firstly, they were already constructed, greatly reducing input of time and planning into the project, secondly, the stone frames would help produce additional reflective heat, thirdly the removable glass covers to the frames would provide a protection shield from any cool or rainy summer weather. Given the weather of summer 2021, the glass covers have been not just helpful, but essential to our melon growing experiments. As for where to source the manure and straw, a neighbouring stable was more than happy to provide us (in exchange for a melon, if the experiment proved successful) with plentiful amounts of stable straw and fresh manure.
We began building the beds in March 2021. Within the three cold frames, my co-worker Jack Griffin and I layered alternately fresh horse manure and stable straw. We kept the layers thin and stacked them up to the lower lip of the frames, about 80cm deep. We then covered the dung-compost with black sheeting and the glass covers to kick off the decomposition process. At intervals I would water the mixture to aid the process. We could quickly see that the process was working as the layers began to shrink down into the frames.
In the meantime, I sowed the cultivars we would be growing for the experiment: Minnesota Midget, Petit Gris de Rennes, and Sugar Baby. Minnesota Midget and Petit Gris de Rennes are both muskmelons bred for cooler and shorter growing season. Sugar Baby is a leap of faith as it is a watermelon, albeit a small, early ripening cultivar of watermelon. Watermelons have been in cultivation for at least 5,000 years and the earliest confirmed documentation of muskmelons dates to the 3rd century BC; the long history of cultivation and breeding has produced a diverse range of melons providing us today with cultivars that are suited to the shorter and cooler seasons we have in higher latitudes. (Of note, melon plants dislike too much root disturbance, so we avoided this by sowing individual seeds into 9cm pots, allowing the plants to grow undisturbed until it was time to plant them on in the hotbeds.)
At the end of May, the melon hotbeds had shrunken by about 30cm and we refilled the bed to the bottom lip with topsoil. With little fear of frosts returning in the first week of June, the young melons were planted, two plants per cultivar per hotbed. Throughout the growing period we have had to alternate between the glass covers being open or closed; there have been very few evenings this summer in which we have left the covers off. When the glass covers are closed, we use bricks to keep them propped slightly open to allow air flow and access to pollinators. To further attract pollinators, Queen of Sheba basil transplants were planted in the hot beds alongside the melons.
As all our seedlings germinated and matured into healthy plants, we opted to cultivate a control group with the excess transplants to compare against our melon hotbed experiment. In individual pots in our polytunnel, as the control group, we grew two of each cultivar planted in an all-purpose growing medium containing slow-release fertilizer.
By the end of July, fruit had begun to set on all our melon cultivars in the hotbeds. We cut back excess fruitless vines, pinched out the growing tips of fruit bearing vines, and lifted the set fruit onto slabs of slate and stone (to protect from soil dampness and provide additional reflective heat). We also cut back on watering entirely as lack of water helps the fruit develop sweetness and avoid cracking; looking out for any wilting as a cue to water. At the time of trimming, we counted 19 Minnesota Midget, 8 Petit Gris de Rennes, and 2 Sugar Baby watermelons.
On the 16th of August, we harvested our first Minnesota Midget melon, the size of a tennis ball with a delightful sweet flavour. Since then the other 18 Minnesota Midget melons, of varying sizes, have ripened at intervals, their sweet smell wafting up from the frame. We have found that while the muskmelon flavour is present in each, unfortunately not all have been sweet, especially those of a larger size. This is likely due to the summer weather’s lack of heat and sun.
Moving into September, the Petit Gris de Rennes muskmelons have begun to ripen and the Sugar Baby watermelons have begun to show signs that they are nearing maturity as well.
The Petit Gris de Rennes melons have been a very welcome surprise as each melon has been sweet and unbelievably juicy.
Although, with the recent heatwave, we watered our melons for the first time since the beginning of August and unfortunately this did result in some cracking amongst the Petit Gris de Rennes muskmelons. Thankfully most have ripened within a day or so of doing so and have still been delicious.
At this time, we have still yet to harvest any fruit from our control group of melon plants. The control group plants did set fruit, although quite less, especially in the case of Minnesota Midget, and the fruits of Petit Gris de Rennes and Minnesota Midget are notably smaller in size than their hotbed counterparts. The Sugar Baby plants have each set a melon, two total. We have found that as the evening temperatures have begun to taper off, some of the control group’s set fruit have begun to die off. This may be due in part to – and this is important – a significant difference in soil temperature between the hotbeds and polytunnel pots. When pushing a finger down into the hotbed topsoil a noticeable warmth and humidity is detected emanating from below the soil, whereas comparably no warmth is detectable emanating from the growing medium in the control group pots (as such the control group plants and their fruit are more exposed to cooler nights).
Overall, it has been a very exciting and fun trial of growing melons here at Parham and one that we are keen to continue in subsequent seasons. Given the success of the melons, we are considering what other heat-loving produce could be cultivated using the hotbed technique and, as we begin to turn towards the autumn and winter months, we will be experimenting with growing winter greens in the hotbeds once the melons are done producing. If successful, it will only expand the valuable uses of hotbeds at Parham. (And as promised, we have made sure that our neighbour has received a Petit Gris de Rennes muskmelon in gratitude for their contribution of manure and straw).