Table of Contents
Greenhouse onion farming gives the crop a roof. That one change removes two of the biggest risks in the open field. Rain spreads disease, and thrips arrive with the wind. Onions still need the same shallow, steady moisture, the same spacing and the same careful harvest. This guide takes the crop from variety choice to curing, and it gives the figures a Kenyan grower needs for an 8 by 30 metre house.
A greenhouse does not guarantee a good onion crop. In practice, heat builds under the polythene faster than many farmers expect, and onions slow down when the house runs above 30 C. The cover helps most where the open field loses a crop to rain, hail or thrips. It helps least where the site is hot and poorly ventilated. Therefore, plan the ventilation before you plan the planting date.
Why Greenhouse Onion Farming Works in Kenya
Kenya grows onions in two main windows. The rains decide the quality of both. Rain on a maturing onion crop splits bulbs and stains the skins. Wet leaves invite purple blotch and downy mildew. A roof removes that risk. Greenhouse onion farming also blocks the wind that carries thrips into a field. Thrips feeding opens the leaves to disease and cuts yield.
The trade-off is cost and heat. A greenhouse costs more per square metre than an open field. Therefore, the crop inside must earn a premium through cleaner bulbs, off-season supply or a seed crop. Heat is the other limit. Typically, onions grow best between about 15 and 25 C. Above 30 C the plants slow down and bulbs stay small. A house with open sides, a high ridge and good airflow holds a workable temperature on most Kenyan sites.
| Factor | Open field | Greenhouse |
|---|---|---|
| Rain on the crop | Uncontrolled, and the main cause of split bulbs | Blocked, so harvest timing is yours |
| Thrips pressure | Constant, carried in on the wind | Lower at entry, but the population builds once inside |
| Disease spread | Fast after rain | Slower if the foliage stays dry |
| Heat | Moderated by wind | Trapped unless the sides and ridge vent well |
| Cost per square metre | Low | High, so the crop needs a premium |
What changes when an onion crop moves under cover.
Step 1: Pick an Onion Variety That Suits the House
Onion varieties respond to day length. That rule decides what will bulb in Kenya. The country sits on the equator, so day length stays close to 12 hours all year. Short-day and intermediate varieties bulb under those conditions. Long-day varieties from Europe need 14 hours or more and will grow leaves without forming a proper bulb.
For example, Bombay Red and Red Creole are the red short-day types most Kenyan growers know. Both sell well in local markets. Texas Grano and Neptune F1 are white and grano types that suit buyers who want milder bulbs. Super Yali is a lowland option. Within that list, choose for bolting resistance and skin quality. A variety that runs to seed after a cold spell wastes the whole house.
Market matters as much as agronomy. Red onions dominate many local markets, while hotels and supermarkets often pay more for white or grano types. Therefore, plant what you can sell before you plant what grows best. Variety choice sets the ceiling for greenhouse onion farming, because the house cannot correct a plant that will not bulb.
Step 2: Raise Seedlings for Greenhouse Onion Farming
Onion seed establishes slowly. Most growers raise seedlings in a nursery and transplant them. A seed rate of 3 to 4 kg per hectare is the working figure. An 8 by 30 metre house covers 240 square metres, which is 0.024 hectares. It needs about 80 to 100 grams of seed.
First, sow in trays or in shallow beds inside the house, in a fine, well-drained mix. Also, keep the surface moist but never wet, because damping off kills seedlings in a closed, humid space. The seedlings are ready at about six to eight weeks. By then they have four or five true leaves and stand around 15 cm tall. Finally, harden them for five days by watering less before transplanting.
Greenhouse onion farming has an advantage at this stage. The nursery sits under the same roof as the final crop. Seedlings avoid the rain damage and insect attack that a field nursery suffers. It also removes the transport shock that knocks back bare-root seedlings moved from an open bed.
Step 3: Set Out the Beds and Spacing
An 8 metre width fits six 1 metre beds. Paths of about 40 cm separate them. Then run the beds the length of the house, so each one is 30 metres long and covers 30 square metres. Six beds give 180 square metres of planted ground inside a 240 square metre house, and the rest is path.
Next, set three drip lines on each bed at 30 cm spacing, and plant onions 10 cm apart along each line. That gives about 900 plants per bed and roughly 5,400 plants per house. The spacing matters for bulb size, because onions planted closer produce more bulbs of lower grade. Also, keep the lines about 15 cm from the bed edge so the outer rows still wet properly.
Then lay the tape on the bed surface under a light mulch, or no more than 5 cm below it. Deep tape wastes water below the shallow root zone and gives poor wetting at the surface where the young roots feed. Finally, test each line for flow before transplanting, and flush the run before the seedlings go in.
Step 4: Irrigation and Fertigation for Greenhouse Onion Farming
Onion roots reach only about 0.4 metres, so the crop cannot chase water deep into the soil. The FAO crop coefficient table for onion puts the mid-season figure at 1.05, with a season of 150 to 210 days. The crop therefore uses close to its full evaporation demand once bulbing starts. At 5 mm of daily demand, that is about 5.25 mm of water a day. The shallow root zone means it must arrive in small, frequent doses.
For greenhouse onion farming, run the drip for short cycles, once or twice a day in hot weather. Check moisture at 20 cm, not at the surface. The critical period runs from bulbing until the necks soften. A dry spell then gives small bulbs, and a wet spell gives split skins. Reduce water in the last two to three weeks before harvest, once the tops begin to fall.
In addition, fertigation suits this crop because the nutrients go straight to the root zone. Apply about 100 to 150 kg of nitrogen per hectare over the season, split between transplanting and bulbing. Add phosphorus at planting and potassium through bulb formation. Stop nitrogen three to four weeks before harvest, or the bulbs grow thick necks that store badly.
Also, filter the water to 120 mesh and flush the lines monthly. Onion crops run for five months or more, which is long enough for algae and silt to plug an emitter. A drip irrigation system in Kenya for a house this size needs a small pump, a filter and a valve per bed group. That page covers the design.
Step 5: Manage Thrips and Disease Under Cover
Thrips are the main insect problem. They hide in the folded leaves and suck sap, leaving silvery streaks and black specks of droppings. Feeding damage also opens the leaf to purple blotch, which then spreads through the house. Therefore, scout at least five plants from different beds and check the leaf bases, not just the top leaves.
Also, yellow or white sticky traps show when adults arrive. They give an early warning before damage appears. Therefore, once the count passes about three thrips per leaf, treat. Rotate between active ingredients so the population does not build resistance. Avoid broad-spectrum sprays that kill the predators which would otherwise help.
Sanitation carries more weight in greenhouse onion farming than in the field. Remove volunteer onions and weed hosts, clear crop debris between cycles, and never transplant seedlings from an infested bed. Also, keep the sides open during the day so humid air leaves the house. Avoid evening irrigation, because a wet canopy overnight is what downy mildew needs.
However, overhead watering is sometimes recommended in the field because it washes thrips off the leaves. Under a greenhouse roof that trade is a poor one. The water raises humidity, wets the foliage and spreads disease faster than it removes insects. Drip is the better choice inside the house.
Step 6: Harvest, Cure and Store the Bulbs
Harvest when about half the tops have fallen. The necks should feel soft. Stop irrigation two to three weeks earlier, then lift the bulbs on a dry morning and leave them to dry. Next, cure them for 10 to 21 days in a dry, shaded, ventilated place. Cut the tops 2 to 3 cm above the bulb, and trim the roots without cutting into the base.
Then grade by size and skin quality before storage. A bulb with a thick neck or a wet cut will rot in the bag. It can take the rest with it. Finally, store in a cool, dry room with airflow, in shallow crates rather than deep sacks, and inspect weekly. Well cured onions hold for two to three months. That lets you sell into a price gap instead of at harvest.
What a Greenhouse Onion Crop Returns
The gross return for greenhouse onion farming starts with plant count. An 8 by 30 metre house holds about 5,400 plants. A well-managed bulb weighs 150 to 250 grams, so the house yields roughly 800 to 1,350 kg. At a farm-gate price of KSh 50 to 80 per kilo, the gross return is about KSh 40,000 to KSh 108,000 per cycle.
However, that figure is gross, not profit. Seed, fertiliser, thrips control, labour and the greenhouse itself all come out of it. A poor price at harvest can cut the top end. Use the model to test your assumptions, not as a promise. The published cost of a drip system in Kenya helps with the irrigation side of the budget.
Typically, two cycles a year are possible with a short break for sanitation. That is one reason the house earns its cost faster with onions than with a single long-season crop. Also, keep the nursery and the bulbing crop staggered, so income arrives in smaller, more frequent lots.
Mistakes in Greenhouse Onion Farming
- Planting a long-day variety. It grows leaves without bulbing at the equator. Choose a short-day or intermediate type.
- Sowing too thick. Crowded seedlings stay thin and never size up after transplanting.
- Burying the tape deep. Onion roots are shallow, so deep tape wastes water below them.
- Keeping the house closed on hot days. Temperatures above 30 C slow bulbing and stress the plants.
- Using overhead water under cover. It raises humidity, wets the leaves and spreads disease.
- Late nitrogen. Feeding close to harvest gives thick necks that rot in store.
Frequently Asked Questions About Greenhouse Onion Farming
How long does an onion crop take in a greenhouse? About five to seven months from transplanting, depending on variety. The FAO season lengths for dry onions run from 150 to 210 days.
How much seed does an 8 by 30 metre house need? About 80 to 100 grams. That is based on a seed rate of 3 to 4 kg per hectare and 240 square metres of house.
Can onions grow in a greenhouse in hot areas? Yes, if the sides and ridge vent well and the cover is high enough. Above 30 C the crop slows, so ventilation decides whether the house suits the site.
Do I need drip irrigation for greenhouse onion farming? It is not the only option. It is the one that keeps the foliage dry while feeding the shallow roots. Overhead water inside a house raises humidity and disease pressure.
How many onions does one greenhouse hold? About 5,400 plants at 10 cm by 30 cm on six beds of 1 metre by 30 metres.
What yield should I expect? Roughly 800 to 1,350 kg per cycle under good management, based on 150 to 250 grams per bulb.
For a house design, send the site, the house size and the water source. Eunidrip Irrigation Systems designs and installs the system and supplies the drip kit for the beds. Call 0728 163329 or email info@eunidripirrigationsystems.com. The onion drip irrigation kit in the online shop shows the components for a block of this size. The greenhouse guide covers the structure itself.