A greenhouse solves temperature, water and wind, and in solving wind it creates a problem that catches most new growers in their first season: flowers open, look healthy, and drop without setting fruit. The plant is not sick and it is not underfed. The pollen never moved.
Outdoors, two forces move pollen constantly. Wind shakes flowers, and insects visit them. A closed structure removes the first almost entirely and greatly reduces the second. For crops that depend on either, someone has to substitute.
Which crops need what
| Crop | Flower type | What it needs | Method |
|---|---|---|---|
| Tomatoes | Perfect flower, self-fertile | Vibration to shake pollen loose inside the flower | Flick or tap the truss daily, or use a vibrating tool at the stem |
| Peppers and eggplant | Perfect flower, self-fertile | Light vibration and air movement | A circulation fan is usually sufficient, plus an occasional tap |
| Cucumbers, standard varieties | Separate male and female flowers | Pollen physically carried between flowers | Brush or a picked male flower onto the female stigma |
| Cucumbers, parthenocarpic varieties | Female flowers set without pollination | Nothing at all | Choose these for a greenhouse and skip the whole issue |
| Squash, melons, courgette | Separate male and female flowers | Pollen physically carried between flowers | Male flower onto female stigma, mid morning |
| Strawberries | Perfect flower | Thorough distribution across the whole receptacle | Soft brush over every flower, or a fan, since uneven pollination causes misshapen fruit |
| Beans and peas | Self-pollinating | Nothing | They set inside a closed flower before it even opens |
| Leafy greens and roots | Harvested before flowering | Nothing | Pollination is irrelevant unless saving seed |
The most useful row on that table is the parthenocarpic cucumber. Varieties bred to set fruit without pollination are widely available and specifically intended for greenhouse growing, and choosing one removes the daily job entirely. They are usually described as all-female, seedless or greenhouse varieties. The one caveat is to keep them away from a pollinated variety, since cross-pollination produces bitter, misshapen fruit.
Temperature is the limit, not technique
This is the part that reframes the whole subject. No amount of good pollination technique helps if the pollen is not viable, and pollen viability is bounded by temperature at both ends.
For tomatoes, pollen release fails below about 55 degrees Fahrenheit at night and pollen is denatured above about 95 by day. Between roughly 55 and 60 at night, fruit sets slowly and often misshapen. That is a working band of about 40 degrees, and it is narrower than most other things a tomato plant can tolerate.
Humidity matters too, though less sharply. Below about 40 percent relative humidity, pollen dries and does not adhere to the stigma. Above about 70 percent it clumps and does not release cleanly from the anther. The band in between is comfortable, and the humidity and VPD chart puts it in context.
How to do it
For tomatoes, peppers and eggplant, the job is vibration. Flick the flower truss sharply with a finger, or tap the supporting string or stake, once a day in mid to late morning when pollen is dry and loose. A visible puff of pollen from a truss on a bright morning means it worked.
At any scale beyond a few plants this becomes tedious, which is why a USB Rechargeable Vibrating Pollination Tool ($25.99) exists: a small vibrating wand touched to the stem behind the truss for a second or two does the job faster and more consistently than a finger. For a larger house, a Electric Pollination Sprayer for Fruiting Crops ($160.00) covers a full row quickly.
For cucurbits, the job is transport. Male flowers appear first and in greater numbers, on a plain stem. Female flowers have a tiny immature fruit behind the petals, which is unmistakable once you have seen one. Pick a freshly opened male flower, peel back its petals, and brush the anther directly against the stigma in the centre of the female flower. A soft brush works equally well and lets one male flower serve several females.
Do it in mid to late morning on the day the female flower opens. Female cucurbit flowers are receptive for a single day, which is the detail that catches people out: a female flower missed is a fruit lost, and the small fruit behind it yellows and drops within a few days.
For strawberries, the job is evenness. A strawberry is an aggregate fruit and each seed on its surface corresponds to a separately pollinated ovule, so partial pollination produces the classic misshapen, hard-shouldered fruit. A soft brush passed over every open flower, or a fan providing constant gentle movement, gives the coverage needed.
Letting insects do it instead
The alternative to doing it yourself is inviting the pollinators in, and in a hobby greenhouse this is often the better answer.
- Open the door and vents on warm days. Bumblebees in particular will find their way into a greenhouse and are excellent tomato pollinators, because they buzz-pollinate: the vibration frequency they generate is far more effective at releasing tomato pollen than any flick.
- Grow flowers among the crop. A few pots of open, single-flowered annuals near the door draw pollinators in and keep them coming back.
- Do not screen the whole house. Insect mesh such as Ultra-Fine Garden Mesh Insect Netting, 10 x 50 ft ($27.99) keeps pests out and pollinators out equally. Screening a vent is sensible; screening every opening on a house full of fruiting crops trades one problem for another.
- Be careful with sprays. Anything applied to a flowering crop under cover reaches pollinators in a concentrated, enclosed space. Follow the label on any product used on an edible crop, and treat outside flowering hours where the label allows.
When pollination is not the problem
Two other causes of flowers dropping are worth ruling out before spending time with a brush.
Excess nitrogen pushes a plant into vegetative growth at the expense of fruit, producing large dark foliage and few flowers that hold. The fix is switching to a high-potassium feed once the first truss sets, and feeding less than an outdoor label suggests in a greenhouse bed that never leaches.
Water stress at flowering causes abortion regardless of pollination. A plant that dries out during flowering drops flowers as a survival response. This is one of several arguments for a timer and drip line, covered in watering in a greenhouse.
Pollination equipment
Pollination sits inside the wider temperature question, because the working band for pollen is narrower than the band the plant itself tolerates. Set the shade and the heat against that band using cooling a greenhouse in summer and heating a greenhouse in winter.