Drivhusklubben/Greenhouse Forum/Gewächshausclub

03 Apr 2024 12:26

Guide: Succeed with Melons

Most greenhouse owners grow tomatoes, cucumbers, and chili as their eternal classics. These crops are certainly cultures that everyone benefits from, no matter the greenhouse type. For instance, the range of tomato varieties is so extensive that there's a variety for every greenhouse model, from tiny dwarf tomatoes that thrive in a city balcony greenhouse.

With melons, it's a bit different. Melons require space and plenty of warmth in a greenhouse. In the coming months, you can learn to cultivate melons in your greenhouse. Month by month, we'll provide tips and tricks so that you can add a new favourite alongside tomatoes, cucumbers, and chili.


The first spring month has started, and many have likely begun pre-growing both tomatoes and chili. Here's the first, perhaps best, tip for growing melons...

WAIT! Wait around 4 weeks before starting with planting soil, seed trays, and seeds on the windowsill. In March, there are other elements that are important to consider. A good start forms the foundation for success throughout the summer season.

The good advice

  • Wait with pre-growing until April.
How much space do you have?

Melon cultivation requires space. Unlike tomatoes, which come in dwarf and semi-dwarf varieties, all types of melons take up a lot of room in the greenhouse. Due to the climate during the summer, we wouldn't recommend growing melons in greenhouses that are 6 square meters or smaller. At a minimum, you should have a greenhouse that is 10 square meters or more to truly enjoy melon cultivation.

The growth of melons is very similar to cucumbers, and for those familiar with this growth, they know how lush, wild, and rapid the growth is during the summer months.

The melons we know naturally have a crawling or creeping growth. Melons can be cultivated both horizontally and vertically. With horizontal cultivation, allow the plant to grow at the bottom of the greenhouse. The effort to control the plant's growth isn't particularly significant with this method. This method requires the most space but also demands the least work.

With vertical cultivation, you can tie up the plant and let it develop fruits along the tied main stem. This allows better utilization of the greenhouse, but it requires a lot of tying up of both plants and fruits. However, it's space-saving and theoretically can yield more fruits.

Regardless, you should not have more than one plant per 8 to 10 square meters. If you choose otherwise and grow several plants per square meter, you might encounter challenges with excess moisture in the greenhouse during the summer.

The good advice
  • Best utilization of the plant is with vertical cultivation.
  • Horizontal cultivation requires less work.
  • The number of plants and greenhouse size should match.
What type of melon? 

The art of limitation can be challenging, especially when one is eager to try various things in their greenhouse. The melon family is a large one with many members. Melon, Cucumis melo, belongs to the Cucurbitaceae family, also known as the squash family. Melon is related to many other well-known vegetables such as squash, cucumber, pumpkin, and watermelon. In this article, we're addressing all types of melons and watermelon as a whole. 

Before you buy with your heart, let's think it through. Originally, melons aren't native to England. Melons have been cultivated for the last 4000 years (approximately 2000 BC) in Persia and Africa. The climate there is entirely different from our part of the world. 

The major challenge in our climate zone is that we lack summer length even though we cultivate in the greenhouse. Before you choose the varieties, you want to grow, you should look at the development time from start to mature fruit. This is usually listed on the back of the seed packet. Choose varieties with a short development time, preferably suited to your local climate. 

For instance, if you fall in love with the taste of a melon on a trip to Southern Europe and bring seeds home, don't expect success. The type of melon is undoubtedly adapted to the climate where it's grown. This could mean that when summer ends in your greenhouse, the fruits might still be far from ripe. 

In our experience at Spirekassen, Charentais melon matures nicely as the Danish summer transitions into autumn. However, watermelon 'Sugar Baby' might struggle to ripen before summer turns into autumn in Denmark. There can be local differences from country to country and between different varieties. 

The good advice 

  • Buy your seeds locally. 
  • Check the development time on the back of the seed packet. 


April tempts with its spring sun and many warm hours in the greenhouse. Perhaps you think you're behind if you haven't pre-grown anything by Easter, but you still have time. In fact, when it comes to melons, you're far from behind but right on time. Melons are among the last things you should pre-sow on the windowsill in April. A basic rule is to sow 3 to 4 weeks before transplanting into the greenhouse. Once the melon seed has sprouted, the plant develops rapidly. Within weeks, it's the plant that dictates in your windowsill. Therefore, you should start pre-growing so late that the plant can naturally be moved out when it becomes too large. 

Ready for pre-germination – equipment, etc. 

You can spend a lot of money on equipment for pre-germination, but there's not much reason for it. It's much smarter to choose the right equipment from the start and reuse it year after year. When selecting pre-germination equipment, consider a few but essential parameters. 

  • What type of seeds am I growing? 
  • What time of year should I pre-germinate? 
  • Is the species' growth fast or slow? 

A melon seed is a large seed, and with the right sowing and temperature, it sprouts quickly, often within 7 days. 

It's essential to choose a seed tray with a relatively large soil volume and ample space between each plant cell. Therefore, avoid small compact plug boxes. They're often designed to maximize space utilization in the windowsill but aren't suitable for melon cultivation. 

Some simple skip pre-germination trays  and start pre-germination in an ordinary 12 cm pot. This is actually the solution we at Spirekassen choose. 

Your seeds should always be sown in a seed and pricking soil. Seed and pricking soil is a very mild soil with not much fertilizer, often fine and porous. The soil is tailored to the sprouting seed and very small delicate plants. 

Make the soil moist and create a small hole for the seed. The seed should be covered with soil no more than three times its thickness. A melon seed is about 3 mm thick and should therefore be sown at a depth of about 0.5 cm. Sow the seed vertically, not horizontally. This can prevent rot in the seed if you tend to overwater. 

The good advice 

  • Choose a seed tray with ample space between each plant and a large soil volume. 
  • Invest in quality that can be reused year after year. 
  • Always use seed and pricking soil. 
  • We recommend not using seed and pricking soil that is compost-based. 
Climate conditions during pre-germination 

Cover your seed tray with a plastic lid, plastic bag, or fleece. The best is to place the seed tray in an east- or west-facing window. The optimal germination temperature is 22-25 degrees Celsius. When the first set of true leaves appears, remove the cover, as the delicate seedlings need to learn about the real world. The small seedlings need as much sunlight as possible. 

If the plants get too much heat combined with too little light, they'll grow too tall and leggy. There will be a risk of them breaking or falling over. Reduce the heat slightly when the plants have developed their first set of true leaves. Windowsills can get quite warm in April. 

Many uses artificial grow lights for pre-germination. You don't need that for melons if you start your pre-germination in April. By that time, the natural light has increased so much that investing in grow lights would be a waste of money. Soon, the plants will need to visit the greenhouse. 

The good advice 

  • Use as much natural light as possible. 
  • Skip artificial grow lights if you start pre-germination in April. 


The small plant 

When your plant is between 3 to 4 weeks old, it's time to start adding liquid fertilizer to the watering water. Many mistakenly believe that fertilizer is something added right from the start of pre-cultivation. It's not. The sprouting seed carries a small energy reserve within itself. It's called endosperm. The endosperm contains carbohydrates, proteins, and fats. Enough for the little seedling to have nutrients in the first weeks of life. Fertilizer is added 3 to 4 weeks after the start of germination, by which time the plant typically has 1 to 2 sets of ‘true leaves, also known as foliage leaves. Add fertilizer 1 to 2 times weekly in the watering water. 


If you haven't initially sowed in large pots but in germination trays, it's time for repotting your small melon plants. Melon is a delicate plant and not nearly as robust as, for example, a small tomato plant. Especially vulnerable is the root collar. The root collar is the last part of the stem, closest to the soil. It's the portion just before the stem becomes a root. When repotting, it's important that the root collar remains above the soil surface and is not covered with soil. Some continue to use seed and pricking soil during repotting, while others use potting soil. Potting soil is more fertilized than seed and pricking soil. Therefore, you shouldn't add fertilizer to the watering water for a few weeks after repotting. 

When the plant is 3 to 4 weeks old, it needs support from a plant stake. 

The good advice 

  • Time for more space - time for repotting
  • Don't cover the root collar with soil.
  • Remember support with a plant stake. 
A visit to the greenhouse 

It might be tempting to transplant your melon plants on a warm day in May in the greenhouse. Even if you're eager, you shouldn't do it yet. Two elements need to be fulfilled: 

  • The perfect greenhouse climate 
  • Plant acclimatization 

The young melon plants need to slowly get used to the spring sun and the greenhouse climate. Start by placing your plants in the greenhouse for 2 to 3 hours of warm weather. But don't put them in direct sunlight; place the plants in the shadiest spot in the greenhouse. If you expose your plants to direct sunlight in the greenhouse, they might get burned, just as we get sunburned. 

The next day, the plants can spend a little longer in the greenhouse, and so on. After about 1.5 weeks, the plants are ready for relocation. 

However, it's important to have measured the nighttime temperature in the greenhouse. The temperature should not drop below 15 degrees Celsius at any time of the day. Small melon plants are very vulnerable to cold. 

The good advice 

  • Gradual acclimatization to the greenhouse climate 
  • Minimum 15 degrees Celsius round the clock 


Planting in the greenhouse 

If you've been very prudent and waited to transplant until early June, you're on the safe side regarding planting. Many plant in May, but often, the plants perish. They frequently collapse due to fungus in the root collar. June is a month characterized by the plant establishing its root system well in its permanent location. Following this, an almost explosive growth begins. 

Planting in the greenhouse 

You probably know that tomatoes can be planted deep in the soil without harm. With melons, it's the opposite. Melons are more susceptible to soil-borne fungal diseases. Melons must not be planted so deep that the new soil enters and covers the stem and root collar of the plant. It's important to choose the warmest and sunniest location for your melon plant. 

Of course, we want as many fruits as possible on our melon plant, and here are a few tricks you should know. It's simply about not being too generous to your melon plant. You shouldn't grow your melon in self-watering containers for the simple reason that the plant has too easy access to a lot of good nutrients and water. If a melon plant has these in abundance, the plant tends to produce a lot of leaves but fewer flowers. This is referred to as the plant being in the vegetative stage. 

Instead, the aim is to move the plant into what is called the generative stage, where it produces flowers, fruits, and seeds. The plant needs a bit of stress, such as short periods of drying between waterings, to induce this generative stage. Another way to encourage the generative stage is to hold back a bit on fertilization. 

If you still cultivate in self-watering containers or pots, don't expect a large yield, perhaps only 3 to 4 fruits per plant. The best option is planting in a fixed soil bed. Here, you can expect around 10 fruits per plant. 

Temperatures in the greenhouse 

Even though melons grow best at 28–35 degrees Celsius, the plant doesn't tolerate temperatures above 40 degrees Celsius for very long. Most fruits develop when the temperature is between 18.3–23.9 degrees Celsius during the day and not below 15.6 degrees Celsius at night. Fruit set is best between 18.3–23.9 degrees Celsius. This is when pollinating bees are most active. 

The good advice 

  • Don't plant too deeply - keep the root collar free. 
  • Plant in the warmest place in the greenhouse. 
  • Avoid self-watering containers. 
  • Allow the plant to slightly dry out between waterings. 


Caring for melons in the greenhouse 

The care routine for melons can vary depending on how you cultivate them. If you're training your melon plants vertically and using trellises, it's a good idea to remove side shoots, much like you would with a tomato plant. When the fruits start forming, it's crucial to provide support for each fruit, as they can become quite heavy. In a horizontal cultivation method, it might be challenging to distinguish side shoots. It's recommended to take a bit of effort every 14 days throughout the season, primarily to ensure the fruits receive adequate air and light. 

Water and nutrients 

Melons, in general, are water-loving crops. The turgor pressure drops quickly if the plant doesn't receive enough water. However, occasionally stress your plants by allowing them to slightly dry out between waterings. This practice encourages more fruit production. 

For those using self-watering containers, follow the instructions for fertilizer application provided on the back of the fertilizer container. Remember to use tap water in the self-watering container and not rainwater. 

In fixed soil beds, it's best to use slow-release pellet fertilizers and apply them 2 to 3 times during the growing season, depending on your soil. 

The Good Advice 

  • Remove side shoots when possible. 
  • Avoid self-watering containers as they often yield lower returns. 
  • Use slow-release pellet fertilizer for fixed soil beds. 
  • Overfertilization results in fewer fruits. 



With cucumbers, the challenges of pollination have long been overcome. It's a bit different with melons. On melon plants, there are both male and female flowers as well as hermaphroditic flowers on the same plant.The female flowers need to be pollinated by pollen from the male flowers before the melon fruit develops. You can tell if a female flower has been correctly pollinated when it forms a small node at the base of the flower, situated beneath it. While you can mimic bees by using a small brush or your hand for pollination, our experience shows that it's rarely necessary. Keep the windows open throughout the summer. Attract pollinating bees into the greenhouse with flowering plants. Sow some nectar-rich plants like Phacelia tanacetifolia or sweet pea, Lathyrus odoratus in the greenhouse, and let nature take its course. The melon flower contains no nectar but only pollen, which is the bees' protein source. The flower is visited by bumblebees, honeybees, and solitary wild bees. 

When is the fruit ripe? 

There's nothing quite as delightful as the taste of a sun-ripened melon. The flavour of the melon fruit is formed by several components, broadly divided into three elements: the ratio of various acids, sugars, and aromatic compounds. A melon fruit consists of 90% water, 9% sugar, and 1% protein. The fruit is ripe when it easily detaches from the plant with a gentle tug. If you need to exert any force, it's not yet ripe. Another trick is to smell the fruit. If you're greeted by an intense melon scent, the fruit is nearly ready. Cantaloupes and Charentais melons get a yellowish hue when they're ripe. 

With watermelons, it's slightly different and a bit harder to determine. On a ripe watermelon, there's a white or yellow spot at the bottom of the fruit. This spot forms when the fruit is pressed against the ground at the bottom of the greenhouse. If you tap the fruit, it should produce a hollow sound. 

For all types of melons, you should feel that the fruit is heavy when you lift it. 

The Good Advice 

  • Keep windows and doors open for bees to enter. 
  • Attract bees into the greenhouse with pollinator-friendly flowers. 
  • Fruits are ripe when they easily detach from the plant. 
  • Ripe fruits are heavy. 
  • Ripe fruits have a sweet and aromatic scent. 


September marks the first month of autumn, and even though we might not feel it yet, the plants in the greenhouse are sensing the arrival of autumn. The daylight hours have decreased, and whether we want it or not, we are heading towards winter. 

Plant life cycle 

Melons are an annual plant. That term, annuals’, says it all - melons are annuals. They sow, develop, flower, fruit, produce seeds, and after that, their life mission is completed, and the plant will die. All of this happens within the same year. The same melon plant won't return next year, but the seeds it produced can sprout and create new plants. 

In September, you'll notice that your melon plants start to look as though they're no longer thriving. The leaves appear dull and start wilting. It might seem like the plant lacks water, but that's not the case. Everything is normal, and you're not doing anything wrong. The life cycle is just coming to an end for the year's melon plant. You likely have a bunch of nice melons left that can be enjoyed in the fall. 

Closing for the year 

By the end of September, it's all over for the year. Many might think it would be smart to collect seeds from their cultivated melons and store them for next year's planting. While that's easily doable, you need to ensure that the variety you initially sowed in early spring isn't an F1 hybrid. 

Seed producers constantly develop new varieties through breeding. It involves evolving resistance to diseases, new colours, thicker skins, and improved taste. We intervene in the natural process by creating F1 hybrids with unique traits. When F1 varieties are developed, two varieties self-pollinate for many generations until they degenerate, akin to what's commonly known as inbreeding. Crossing two inbred plants results in entirely new desirable traits, but ONLY in the first generation, the F1 generation. You'll often see the term F1 on seed packets. If you collect seeds from your F1 plants again, you'll end up with significantly inferior traits from the previously inbred parental generation. 

Hence, only save seeds from melons that you've cultivated that aren't F1 hybrids. 

The Good Advice 

  • Keep old seed packets – they contain important information about, for instance, F1 hybrids. 
  • Store seeds in a dry, cool place with a stable temperature. 
  • Look forward to your new greenhouse year. 

(*) The root neck is the part of a plant that forms the transition between the stem and the roots. It's in a vulnerable position because the moisture constantly fluctuates around it, making it susceptible to fungal attacks.