Watermelons can be grown successfully in containers, flower pots and grow bags if they are provided with key elements they require: warmth, plenty of sunshine, nutrients, moist, but well aerated soil.
There are several reasons why would someone 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 last frost day, this leaves 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 gardener to keep watermelon plants warm until frost danger is gone. Also, this way there is no need for transplanting watermelons on permanent location - 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 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 pH between 6 and 6.8, although they will grow between pH 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 garden with soil that is not suitable for watermelons, consider raised beds or containers/pots/grow bags.
- lack of space: watermelons require plenty of space. Choosing smaller varieties save some space, but container growing enables a gardener to grow watermelons where it would otherwise be impossible (for example, roofs, trellis, wire nets embedded in concrete, lawn etc.).
Of course, there are many other reasons for growing watermelons in containers and one of my favorite 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): Sugar Baby, Orange Sweet, Golden Midget, Crimson Sweet, Yellow Baby, Yellow Doll, Jubilee, Moon and Stars, Jade Star, Golden Crown, Early Moonbeam, Millennium, Solitaire, etc.
Each of these varieties can grow in 5 gallon (~19 liters - lets 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 single compact watermelon plant.
In containers that are for example 1x1x3 feet (0.3x0.3x0.9 m), feel free to sow at least 6-7 seeds and leave only the 3-4 strongest plants to grow and bear fruits.
If you plan on growing larger varieties, consider containers that are 40cm (16 inches) deep and at least 50-60 cm (20-24 inches) wide, with many drainage holes.
Again, this depends on chosen variety and the number of planed plants per container.
When plants start to grow, let the vines spread around or grow vertically on trellis or wire mash/net. Small fruits should be protected from touching wet soil and if they grow vertically, grow them in hammock made out of panty hose, t-shirts or plastic mesh fruit bags.
Growing Medium, Watering and Fertilization
When choosing proper growing medium, the best medium is general potting soil mix that can be found in many garden centers - it is slightly acidic and drains excess water well, but keeps the moisture. Often, such potting soil mixes are rather low on nutrients (to promote root growth), so be sure to check the labels. Anyway, be sure to add some organic fertilizer like dried cow/horse/chicken manure (pellets), compost/humus and NPK fertilizer like 15:15:15 with gradual release of nutrients - watermelons are hungry plants, but too much of fertilizers can cause root burns.
Also, watermelons tolerate rather well raw manure and adding it in small amounts (mixed with straw or peat moss etc.) can increase temperature around roots leading to larger harvest. This mix doesn't smell well, so if you are not 100% sure what are you doing, skip it!
Watering of plants grown in containers must be done on a daily basis and when temperatures are above 30°C (above 86°F) plants must be watered twice per day. If the plants are grown in larger containers, watering can be done once per day, even during summer heat - water the plants in the morning, and if there are wilting in the morning or in the afternoon, water more often.
One of the possible solutions for watering is dripping system that water plants constantly. If you don't have 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 bottle to achieve desired water flow, but any excess water will drain away in any case.
Fertilization during vegetation period can be done using liquid fertilizers (once per week) or fertilizers in pellets with gradual release of nutrients (every months). Such fertilizers and their combinations, can be used to easily change amounts and ratios of nutrients in the soil. Watermelons like nitrogen rich soil before flowering, but higher amounts of phosphorus and especially potassium are required for growth of healthy and well formed fruits. Personally, use fertilizers in pellets with gradual release of nutrients for providing most of the nutrients to watermelon plants and use liquid fertilizers to change ratios of nutrients as required.
Pests and Diseases
Root diseases are avoided using new growing medium and avoiding soil from garden.
Slugs and rodents can damage the fruits and plants. Slugs can be removed by hands and damage by rodents can be avoided by growing the plants vertically or by having a cat. Cat will also play 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 such chemicals.
Growing watermelons in containers, flower pots or grow bags sounds complicated, but it is not - watermelons are demanding plants, but when they are given what they want, they return the favor with large harvest of great tasting fruits.