Gadenin日本語Get the appWhy watermelon needs more pollen than most crops
A watermelon has separate male and female flowers on the same plant, and every seed in the finished fruit is the result of one pollen grain reaching one ovule. A large fruit contains hundreds of ovules, and a female flower needs to be visited repeatedly, by several insects, over the few hours it is open, to fill them all. That is a much heavier pollination load than a tomato or a bean and it is why watermelon fails more often at this stage than anything else in the garden.
The consequence of partial pollination is visible in the fruit. A watermelon that only received pollen on part of its stigma grows unevenly: it swells on the pollinated side and stays narrow and pinched on the other, and it usually stops filling and drops. Lopsided fruit are almost never a nutrient problem, whatever the symptom charts say. They are a pollination problem.
- Male flowers come first Male flowers usually open a week or two ahead of the first females. That gap is normal and is not a sign of anything wrong with the plant.
- Both open for one morning A watermelon flower opens around dawn and closes by early afternoon, and it never opens again. Whatever pollination happens has to happen in that window.
- Females are outnumbered There are roughly seven male flowers for every female. Do not read the abundance of males as a fault; it is how the plant supplies enough pollen.
- Seedless types need a pollinator Seedless watermelon produces sterile pollen and cannot set its own fruit. It needs a seeded plant flowering nearby, and without one it will flower all summer and fruit nothing.
Telling male and female flowers apart
The difference is at the base of the flower and it is unmistakable once you have seen it. A female flower has a miniature watermelon, the size of a small marble, behind the petals before the flower even opens. A male flower has nothing but a plain thin stalk. You do not need to look inside the flower at all, though if you do, the male carries a single pollen-covered stamen and the female a lobed, sticky stigma.
| Feature | Male flower | Female flower |
|---|---|---|
| Behind the petals | A plain thin stalk | A tiny fruit the size of a marble |
| In the center | One stamen dusted yellow | A lobed stigma with a moist sheen |
| Where it appears | Everywhere, mostly early on the runner | On side shoots and further out along the runner |
| How many | Roughly seven for each female | One at intervals along the vine |
| How long it lasts | One morning | One morning |
A female whose baby fruit is already yellowing and shriveled at the tip was never fertilized and will not recover. Pinch it off so the plant stops spending on it.
How to hand pollinate, and when
The stigma is receptive from the moment the flower opens and its receptivity falls away through the morning. Aim to be finished by nine in the morning and certainly by ten. After that the pollen itself deteriorates in the heat and even a thorough job sets poorly.
Be far more thorough than you would be with a squash. One male flower per female is the minimum and two or three is better, and the stigma has to be covered on every lobe and all the way around. This is the single point where most home grown watermelon go wrong, and doing it properly turns a hit-and-miss crop into a reliable one.
- Pick two or three males Take males that opened this morning, with an inch of stalk. Pollen from a flower that opened yesterday is spent and transferring it is the same as doing nothing.
- Strip the petals back Peel the petals off the male so the stamen stands clear. Petals in the way physically block the pollen from reaching the stigma, which is the most common way to pollinate without pollinating.
- Cover the whole stigma Roll the stamen gently over every lobe of the stigma and right around it, then repeat with a second male. Aim to see yellow pollen visibly on the surface when you finish.
- Write the date on a label Push a plant label into the ground beside the fruit with the date on it. The harvest decision six weeks later depends entirely on this number, and nothing else records it.
- Repeat the next morning if you can If the same female is still open the next day, which occasionally happens in cool weather, do it again. Extra pollen never hurts and it fills the far side of the fruit.
Fruit that drop at golf ball size
The commonest complaint about watermelon is small fruit that reach the size of a golf ball, yellow at the stalk end and fall off. Almost always this is failed pollination rather than disease, and it looks alarming because the fruit clearly started. The plant is simply aborting what it cannot fill.
There is a second, less obvious cause: a plant carrying more fruit than it can support will shed the youngest ones. If the fruit dropping are the newest sets on a vine that already has two good fruit filling, that is the plant making a sensible decision and there is nothing to fix.
| What you see | Most likely cause | What to do |
|---|---|---|
| Yellows and drops at marble to golf ball size | Never pollinated at all | Hand pollinate every female from tomorrow |
| Swells on one side, pinched on the other | Only part of the stigma got pollen | Use two or three males and cover every lobe |
| Many flowers, no females at all | Too much nitrogen, or the plant is still young | Stop feeding nitrogen and wait; females come on side shoots |
| Females drop while older fruit fill | The plant is already fully loaded | Nothing. Let the older fruit finish first |
| Sets stop entirely in extreme heat | Pollen sterile above 95F (35C) | Keep the plant watered and wait for it to cool |
Getting insects to do the work instead
Hand pollination is a good insurance policy but it is not how you want to spend every summer morning. The lasting fix is to make the planting attractive to bees, because a watermelon flower visited eight or ten times by insects is pollinated far more completely than any human being manages with a stripped male flower.
The two things that matter most are having flowering plants nearby through the whole watermelon season, and not spraying insecticide during flowering. Any insecticide applied to open flowers, even one described as safe, reduces visits for days afterwards. If something must be sprayed, do it in the evening once the flowers have closed.
- Plant flowers on the same schedule Put long flowering annuals at the ends of the watermelon rows so there is something in bloom before, during and after the crop. A steady supply keeps pollinators resident rather than passing.
- Never spray open flowers Hold off any spray until the afternoon or evening when the flowers have shut. Spraying in the morning is spraying the pollinators you need.
- Grow more than one plant A single watermelon plant is a poor draw for insects and gives you no male flowers on the days it makes none. Three plants together set noticeably better than one.
- Record which mornings you intervened Note the days you hand pollinated and the days you left it to insects, then check later which fruit filled evenly. After one season you will know how much intervention your garden actually needs.