How to Grow Watermelons in Containers, Flower Pots, and Grow Bags| The Complete Guide
Watermelons can be grown successfully in containers, flower pots, and grow bags if they are provided with the key elements they require: warmth, plenty of sunshine, nutrients, and moist but well-aerated soil.
Watermelons have shallow roots and can be grown in relatively small containers, pots, and grow bags, but they grow vigorously, requiring a constant supply of nutrients and water.
Updated: March 4, 2023.
Growing Watermelons in Garden vs. Growing Watermelons in Containers, Pots and Grow Bags
There are several reasons why someone would grow watermelons in containers, flower pots, or grow bags:
Growing watermelons in cold areas: watermelons require warmth for proper growth (around 80°F or 27°C are considered ideal) and are very sensitive to frost. Since they should be sown or transplanted outside at least two weeks after the last frost day, this leaves a very short window of opportunity for watermelons to be grown outside in some regions.
But, sowing watermelon seeds in larger containers or pots indoors enables gardeners to keep watermelon plants warm until frost danger is gone. Also, this way, there is no need for transplanting watermelons to permanent locations - avoiding stress is very important not only for humans but for plants, too. Watermelons don't like soil that is below 20°C (68°F), and this can be remedied by covering the soil with thin foil or adding manure to the growing medium to increase both temperature and amount of available nutrients.
Better soil control: watermelons prefer rich, well-aerated, slightly acidic soil with a pH between 6 and 6.5, although they will grow in soil with pH between 5 and 7.
Pots and containers must have many drainage holes since watermelons don't like "wet feet" - waterlogged soil can cause significant issues with roots and prevent successful harvest. If you have a garden with soil unsuitable for watermelons, consider growing the watermelons in raised beds or containers/pots/grow bags.
Lack of space: watermelons require plenty of space. Choosing smaller varieties may save some space, but container growing enables a gardener to grow watermelons where it would otherwise be impossible (for example, roofs, lawns, balconies, terraces, trellis, etc.).
Of course, there are many other reasons for growing watermelons in containers, and one of my favorites is - fun! :)
Watermelon Varieties for Container Growing
When growing watermelons in containers, compact varieties with relatively small fruits are required. These varieties may include watermelons like (Amazon links - links open in the new windows):
Pot/Container/Grow Bag Size
Each of these varieties can grow in 5-gallon (~19 liters - let's say it 20 liters) containers (single plant per 5 gallons of growing medium - often soil-less mix is used for indoor or vertical growing). 5 gallons or 20 liters is not much - for example, 12x12x12 inches (1x1x1 foot, ~30x30x30 cm) containers have 7.4 gallons or 27-28 liters of soil, and that is more than enough for a single compact watermelon plant.
But these are bare minimum values, and when growing plants in containers, especially watermelons, the larger the pot/grow bag, the better.
In containers that are, for example, 1x1x3 feet (0.3x0.3x0.9 m) (20 gallons, ~80 liters), feel free to sow at least 6-7 seeds and leave only the 2-3 strongest plants to grow and bear fruits.
If you plan on growing larger varieties, consider containers 40cm (16 inches) deep and at least 50-60 cm (20-24 inches) wide, with many drainage holes. For example, a 16 x 20 x 20 inches (40 x 50 x 50 cm) container has a capacity of ~25 gallons (~100 liters) and is large enough for 2 (two) small/compact watermelons or one typical watermelon plant.
A flower pot (round one) 16 inches (40 cm) deep and some 20 inches (50 cm) wide has a capacity of ~20 gallons (~80 liters) and is large enough for a single watermelon plant or two compact watermelons.
When plants start to grow, let the vines spread around or grow vertically on a trellis or wire mash/net. Small fruits should be protected from touching wet soil, and if they grow vertically, grow them in hammocks made out of pantyhose, t-shirts, or plastic mesh fruit bags.
Growing Medium, Watering, and Fertilization
When choosing a proper growing medium, the best medium is the general potting soil mix found in many garden centers - it is slightly acidic and drains excess water well but retains plenty of moisture.
Often, such potting soil mixes are rather low on nutrients (to promote root growth), so be sure to check the labels.
Add some organic fertilizer like dried cow/horse/chicken manure (pellets), organic compost, worm castings, and NPK fertilizer like 15:15:15 with gradual release of nutrients - watermelons are hungry plants, and they require plenty of nutrients. Still, too much fertilizers can cause root burns.
Also, watermelons tolerate raw manure rather well, and adding it in small amounts (mixed with straw or peat moss, etc.) can increase the temperature around roots leading to a larger harvest. This mix doesn't smell well, so if you are not 100% sure what you are doing, skip it!
Watering of plants grown in containers must be done daily, and when temperatures are above 30°C (above 86°F), plants in smaller containers must be watered twice per day. If the plants are grown in larger containers, watering can be done once per day, even during the summer heat - water the plants in the morning, and if there is wilting in the morning or in the afternoon, water more often.
One of the possible solutions for watering is a dripping system that waters plants constantly. If you don't have a dripping system, don't worry - just take a large plastic soda bottle, fill it with water, and stick it into the soil. Some trials and errors are required to learn how to position such a bottle to achieve desired water flow, but any excess water will drain away in any case.
Fertilization during the vegetation period can be done using liquid fertilizers (every 2-3 weeks or as the manufacturer recommends) or fertilizers in pellets with a gradual release of nutrients (every 2-3 months).
Such fertilizers and their combinations can be used to change amounts and ratios of nutrients in the soil easily.
Watermelons like nitrogen-rich soil before flowering (too much nitrogen stimulates vine growth without many fruits), but higher amounts of phosphorus and especially potassium are required for the growth of healthy and well-formed fruits.
Personally, use organic and NPK fertilizers with a gradual release of nutrients when preparing the soil and during the plants' growth, and liquid fertilizers to feed the plants quickly if/when required.
Mulch can be added to protect the soil from strong wind and sun, to help retain moisture, and to fight weeds. Also, as the mulch decomposes, it feeds the plants and keeps the soil slightly acidic.
Growing Watermelons In Containers, Pots, and Grow Bags From Seeds
Growing watermelons directly from seeds ensure no stress to plants due to the transplanting.
However, most watermelon varieties require 80-90 days to go from seeds to fully ripe fruits ready for harvest, and in northern zones, the growing season might not be long enough.
If You live in such an area, consider either:
- growing watermelons in containers and pots placed on carts or trolleys, letting the seeds germinate and young plants grow indoors while there is still a danger of cold temperatures outside, or
- grow watermelons from transplants.
So, take a large enough container, pot, or grow bag and fill it with a mix of potting soil and organic and NPK fertilizers.
Plant each seed ~1 inch (2.5 cm) deep and water with stale water. When the seeds germinate and young plants start to grow, remove all but one plant (the best one, of course) per pot/container/grow bag if You have small to medium size pots/containers/grow bags.
Note: If You want more watermelon plants to grow, increase the number of growing pots/containers/grow bags - don't overcrowd the watermelons.
Water the plants regularly and keep the soil well-fertilized.
Growing Watermelons In Containers, Pots, and Grow Bags From Transplants
Since watermelons are very sensitive to cold temperatures, especially frost, they are commonly germinated indoors in smaller plastic pots and transplanted onto a permanent position at least 2 weeks after the last frost date.
Watermelon transplants can be found at garden centers, or the gardener can grow them indoors.
Note: watermelons grow strongly, so having large enough germination pots (one liter or even larger) can be helpful in decreasing the stress to plants.
So, fill the pots with good potting soil, plant 2-3 seeds 1 inch (2.5 cm) deep, and let the seeds germinate.
When the young plants start to grow, remove all but one watermelon plant.
After the danger of frost is gone (2 weeks after the last frost day), plant the young watermelons in their pots/containers/grow bags outside.
Note: always plant the watermelons to the same height they were growing in the (germination) pots. Also, don't disturb the roots if You see them when transplanting the watermelons.
Pests and Diseases
Root diseases are avoided using new growing medium and avoiding soil from the garden.
Slugs and rodents can damage fruits and plants. Slugs can be removed by hand, and damage by rodents can be avoided by growing the plants vertically or by having a cat. Cats also play an important role by defending ripe fruits from birds.
Fungus and other diseases can be treated by water-soluble chemicals based on copper (rarely on sulfur). Such chemicals are easily washed away, and they don't penetrate the fruits' membrane - read the labels even before buying and especially before applying such chemicals.
Watermelons should be harvested when fully ripe, which is often guesswork.
The watermelon is fully ripe when thumbing on the fruit, the watermelon sounds like it is half full.
Also, fully ripe watermelon has tendril half or fully dry.
Watermelons should be picked up early in the morning before the sun warms them.
Served fresh and cooled, they can be a great refreshing treat for hot summer days.
Growing watermelons in containers, flower pots, or grow bags sounds complicated, but it is not - watermelons are water and nutrients high demanding plants, but when they are given what they want, they return the favor with a large harvest of great-tasting fruits.
By choosing different varieties and planting at different times, the watermelon harvest season can be rather long.