Greenhouse Technology – SS2 Horticulture and Crop Production Lesson Note (First Term)

A greenhouse is a structure covered with transparent material, such as glass or plastic, that creates a controlled “micro-environment” for plants.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define a greenhouse;
2. describe the lean-to, even-span and Quonset greenhouse designs;
3. list the essential parts of a greenhouse and state the use of each;
4. state the benefits of using a greenhouse;
5. explain the challenges of greenhouse technology.

What is a greenhouse?

A greenhouse is a structure covered with transparent material (glass or plastic) that traps heat and light to create a controlled micro-environment for plants.

• Purpose: it lets farmers grow crops when the weather outside is too hot, too cold or too dry.
• Everyday example: a car parked in the sun with the windows up feels very hot inside, even when the air outside is cool.
• Same idea: a greenhouse works in a similar way, because the covering lets light in and holds heat inside.
• Crops grown: tomatoes, lettuce, pepper and cucumber are common greenhouse crops.

How does a greenhouse work?

A greenhouse works by letting sunlight in and keeping the warm air inside.

1. Sunlight passes through the transparent covering.
2. The soil, plants and benches absorb the light and give off heat.
3. The covering and closed walls hold the warm air inside.
4. Vents, windows or fans let out extra heat when it gets too hot.
5. The farmer adjusts temperature, humidity and water to suit the crop.

What are the types of greenhouse structures?

There are three common greenhouse designs: lean-to, even-span and Quonset (tunnel).

The design chosen depends on the farmer’s budget, the space available and the plants being grown.

Lean-to design

• Shape: looks like half a house.
• Position: built against the side of an existing building, such as a classroom or house wall.
• Advantage: saves space.

Even-span design

• Shape: the most common “house-shaped” greenhouse.
• Roof: both sides of the roof are the same length.

Quonset (tunnel) design

• Shape: looks like a long half-pipe or tunnel.
• Materials: curved metal pipes covered with a heavy-duty plastic sheet.
• Advantage: cheaper to build, which makes it very popular in Nigeria.

Comparison of the three designs

What are the essential parts of a greenhouse?

A greenhouse has five essential parts: the frame, the glazing, ventilation, cooling and heating systems, and benches.

1. The frame: the “skeleton” made of wood, galvanised steel or PVC pipes.
2. The glazing (covering): the transparent layer that lets light in and holds heat.
3. Ventilation: fans or windows that open to let out extra heat so the plants do not “cook”.
4. Cooling and heating systems: “pad and fan” systems work like a desert cooler to lower the temperature in the hot afternoon sun.
5. Benches: raised tables for pots, so the farmer does not bend down and air can circulate around the roots.

What are the types of glazing?

Glazing materials are glass, polycarbonate and polyethylene film.

Why use a greenhouse?

A greenhouse costs money to build, but it gives the farmer four main benefits.

• Year-round production: fresh tomatoes can be grown in the dry season, or lettuce when it is too rainy outside, because the farmer controls the weather around the crop.
• Protection from pests: the house is closed, so many insects, such as whiteflies and grasshoppers, cannot get in.
• Higher yield and quality: plants grow faster and look “cleaner” because wind and heavy rain do not beat them.
• Water conservation: drip irrigation sends water straight to the roots and less water evaporates, so water is saved.

What are the challenges of greenhouse technology?

The main challenges of greenhouse technology are high cost, overheating, pollination problems and the skills needed to manage it.

• High start-up cost: pipes, special plastic and cooling fans need a lot of money upfront.
• Overheating: in Nigeria a greenhouse can become an “oven” quickly. If the fans fail or the power goes out, crops can die within hours.
• Pollination problems: there are no bees or wind inside a closed greenhouse, so the farmer may hand-pollinate flowers or bring in special bees.
• Management skills: temperature and humidity must be checked every day, which needs more technical knowledge than open-field farming.

Class activity

This activity lets students feel the greenhouse effect in the school garden.

1. Take the students to the school garden.
2. Bring a clear plastic bucket or a large transparent bag.
3. Place it over a small weed or plant in the sun for 30 minutes.
4. Let the students touch the air inside to feel the warmth trapped by the cover.

Evaluation

1. What is a greenhouse?
2. Name the three greenhouse designs described in this lesson and describe one of them.
3. List the five essential parts of a greenhouse.
4. State three benefits of growing crops in a greenhouse.
5. Why may a greenhouse crop die if the power goes out?

Assignment

Explain why the Quonset design is popular in Nigeria. Then describe two challenges a farmer may face when using a greenhouse.