Despite the role gardening plays in my life today, there have been many times when my black thumb reared its ugly head.
I have a sweet spot (no pun intended) for watermelons because they were the first fruit I was able to grow after a summer filled with particularly bad luck!
The watermelon plant — Citrullus lanatus — belongs to the Cucurbitaceae family, which also includes popular garden veggies like cantaloupe, cucumber, and squash. It’s not unusual to see similarities in the growth habits of watermelons and their close relatives.
For the best harvest possible, however, I think it’s important to understand the specific watermelon growth stages.
Conditions for Growing Watermelon
Watermelons are summer growers that need an extended period of warm weather to mature and bear ripe fruit. With proper timing and the selection of appropriate cultivars, watermelons grow well in USDA zones 2 through 11.
One thing you definitely need to know before planting watermelons is that these vining plants need a lot of space to thrive. Plan to allocate 20 square feet per plant within your garden.
If you have the available space, raised beds can be a great way to cultivate happy watermelons. The soil within raised beds naturally heats up faster than the normal ground, which is a great benefit for these warmth-loving plants. Just keep in mind the square footage required per plant.
The best watermelons receive an average of 8 hours of bright sun each day. A lack of good sunlight interferes with healthy flower and fruit development.
Did you know that the average watermelon fruit is 92% water? It’s no surprise then that developing plants need 1 to 2 inches of water per week, depending on weather conditions.
Plant watermelons in soil that is well-draining, loamy, and amended with a large amount of organic matter like compost or aged animal manure. According to the University of Minnesota, the ideal soil pH for watermelon is 6.0 to 6.5.
These plants benefit from lots of nitrogen during the vegetative growth stage — e.g., fertilizer with an NPK ratio of 10-5-5. A formula with high potassium and phosphorus is preferred when flowers start to appear — e.g., fertilizer with an NPK ratio of 5-10-10.
Growth Stages Of Watermelon
On average, watermelons need 70 to 120 frost-free days to bear ripe fruit when grown from seed. Watermelons that require less than 80 or so days to mature are often categorized as short-season varieties.
Note that part of the reason some watermelons ripen faster than others is that they produce smaller fruit. But these plants are still ideal for cool-climate gardeners who want to enjoy fresh watermelons without the risk of early frost damage.
Some popular short-season varieties include:
- ‘Sugar Baby’ (70-75 days)
- ‘Yellow Baby’ (70-75 days)
- ‘Blacktail Mountain’ (70-85 days)
- ‘Cole’s Early’ (80 days)
In my experience, the factors most likely to affect watermelon growth rate (outside of the variety being grown) are moisture, nutrition, and temperature.
Providing consistent moisture will not only improve fruit quality but can also maximize vegetative growth early in the season. In a similar vein, sufficient nitrogen pre-flowering will increase growth significantly.
Warming the soil by even a few degrees using raised beds or plastic mulch can make a big difference in growth rate, especially for us gardeners in cooler climates.
According to Pennsylvania State University, daily air temperatures below 50°F or above 95°F will slow vegetative growth and maturation.
In addition, the growth and overall health of your crop may be enhanced with the help of some watermelon companion plants.
For example, cucumbers and Zucchinis tend to attract the same pests as watermelon so it is best to avoid growing these in close proximity. Instead, try growing lettuce which will help to suppress weeds and carrots to help better utilize garden space as well as radish which will help to deter pests such as cucumber beetles.
Watermelon Plant Timeline – Timelapse Video
1. Seed Germination
Most watermelon seeds are blackish-brown and teardrop-shaped. They will germinate in about 4 to 14 days when temperatures are above 80°F. Conditions above 65°F are needed for watermelon seeds to germinate at all.
Starting any plant from seed normally means sowing the seed in soil and waiting a few days for a sprout to appear. However, there’s a lot that goes on beneath the soil’s surface as part of germination.
Before germination actually begins, water must penetrate the seed coating. This step is called imbibition. The presence of moisture within the seed activates special enzymes that direct germination.
Next, several key structures develop inside the seed embryo. These include the radicle, hypocotyl, and cotyledons.
The radicle is a rudimentary root and the very first part of a plant to emerge from the seed. It anchors the watermelon seed into the soil and takes invaluable resources from the environment.
The hypocotyl and cotyledons emerge shortly after the radicle. The hypocotyl is an embryonic stem — the first stem the plant puts out — and the cotyledons are primitive leaves. These structures use light and gravity to determine which way is ‘up’ in order to break through the soil.
Watermelons are dicots, meaning that each seedling has two cotyledons. These are the only leaves that develop before the seed actually sprouts.
Cotyledons are simple and round — they look much different than the mature foliage of a watermelon plant. While cotyledons are not ‘true’ leaves, they’re capable of some degree of photosynthesis while the watermelon seedling works on producing its adult foliage.
It normally takes 5 to 10 days for a watermelon seedling to grow its first ‘true’ leaves. The cotyledons will naturally fall off within the next couple of days.
As the seedling produces its first adult leaves, it also focuses on sending out a primary vine. At this stage, the vine will grow to about 1 foot long.
All seedlings are delicate but watermelons are particularly so. Great care must be taken when transplanting these seedlings to the garden bed.
3. Vegetative Growth
The watermelon plant will continue growing throughout summer. The main vine can grow up to 12 feet long. New foliage will appear along the vine as it lengthens.
Watermelon leaves are deeply lobed and measure up to 8 inches long. When young leaves first emerge, they are covered in a prickly ‘wool’ but this disappears as the plant matures.
About 30 days after the main vine appears, or about 45 days from planting, lateral vines will begin to sprout.
Pruning back these offshoots may improve productivity. When it comes to my personal experience, though, I’ve always just let my watermelon plants do their own thing.
Most watermelons have a prostrate growth habit. This means that the vines sprawl across the ground. However, most varieties will instinctively climb if provided with strong enough support.
Approximately 60 days after planting watermelon plants begin to flower. Watermelon flowers are yellow and have 5 petals that form a star shape. It’s particularly important to note that watermelon plants produce separate male and female flowers. Male flowers bloom first. Female flowers generally start to appear 10 to 14 days after the initial male flowers.
You can usually distinguish between the two sexes by examining the place where the blossom meets the stem. If there is a small swollen segment of the stem just below the petals, the flower is likely female. This enlarged bit of stem is the flower’s ovary that — if pollinated — will eventually turn into a watermelon fruit.
Flowering will continue as long as the plant itself continues to grow. Many gardeners opt to pinch off new female flowers once the fruit set begins to encourage a bumper crop.
Only female flowers are capable of transforming into fruit. However, you still need the pollen of a male flower to trigger fruit development.
Watermelons are self-fertile, so a female flower will happily accept pollen that came from a male flower on the same plant.
It’s a popular belief that watermelon plants will cross-pollinate with other members of the Cucurbitaceae family and, therefore, must be isolated from related crops. Fortunately, this isn’t true! Watermelons are an entirely separate species from cucumbers, cantaloupes, and other Cucurbits and will not cross with them.
Regardless of sex, all watermelon flowers bloom for a single day. In many cases, the flowers last for only a few hours before wilting. This means that pollination must occur within a very short window.
In addition to the individual flowers only lasting for a few hours, you also need to remember that the male flowers start blooming much sooner in the year than the female ones. Do not be alarmed if your watermelon plants flowers for several weeks before pollination and fruit set occurs.
Honeybees and native bees are believed to be the biggest contributors to watermelon pollination. But there are countless insects and other creatures that can transfer pollen from a male flower to a female one.
In both home gardens and agricultural settings, hand pollination is common practice. This involves wiping pollen from a male flower and transferring it to a receptive female flower. Hand pollination helps guarantee pollen transfer and, in turn, fruit development.
6. Early Fruiting
Once a female flower is successfully pollinated, it only takes a few days for the enlarged segment of stem below the blossom to begin growing.
Female flowers that fall from the vine after blooming are an indication that pollination or egg fertilization was unsuccessful.
Not all fruit produced by a watermelon plant will be perfect. I strongly suggest cutting your losses by removing any fruit that is severely misshapen, physically damaged, or showing signs of disease. Removing unwanted fruit will allow the plant to redirect resources to producing larger fruit elsewhere on the vine.
One of the most common causes of misshapen or low-quality watermelons is poor pollination. Each female flower requires a relatively large amount of pollen to produce good fruit. So, if you’re encountering this problem in your own garden, consider hand-pollinating in the future.
7. Fruit Development and Ripening
Most watermelon varieties require about 30 days to ripen on the vine. During this time, proper care is crucial to healthy, flavorful fruit development.
Since the fruit develops below the actual flower, it’s completely normal for the flower to stay attached to the end of the watermelon as it matures. Over time, the flower will naturally shrivel up and fall off.
Blossom-end rot is an incredibly common disease that causes patches of dark, leathery decay to appear near the flower attachment point. While a calcium deficiency is always to blame for this disease, the deficiency itself can be caused by poor soil quality, inconsistent moisture levels, or imbalanced fertilization.
When To Harvest Watermelon
Begin checking watermelons for ripeness about 30 days after pollination and fruit set occur. There are several ways to check if a watermelon is ready to harvest, including:
- Locate the tendril nearest the fruit. The tendril will be soft and green as the fruit develops but turn brown and dry when it ripens.
- Flick the watermelon rind with a finger and listen for a dull, hollow sound. This means the fruit is probably ripe.
- Examine the rind for proper coloring and a matte finish. Unripe watermelons tend to be shiny.
- Gently lift the fruit and check the weight. Ripe watermelons are usually heavier than they appear.
Watermelons must be left to ripen on the vine — they won’t continue maturing properly once picked.
You can leave most ripe watermelons on the vine for about 14 days before the fruit is likely to split. I prefer to pick watermelons right away, however, to reduce the risk of weather or animal damage.
You may also like to read Lettuce Plant Growth Stages | Lifecycle
FAQ Watermelon Plant Growth Stages
- The University of Minnesota Soil requirements for watermelon
- Pennsylvania State University Ideal temperature for watermelon growth
Ben's horticultural interest grew when graduating from Hertfordshire University in 1997. Having contributed to numerous publications including Better Homes & Gardens, Garden Design Magazine, and The English Garden. He is also the author of Propagating Houseplants Made Easy.