You need to know what you will be growing. Your market will help if not decide what you will or can grow. It can be done but it’s tough to persuade a market into buying a product they haven’t been looking for. I mention this a lot throughout several blog post and it’s because it is the foundation of the greenhouse design.
The size of the greenhouse is based on several things. The first is volume of product you need to grow. How much product do you need to produce for the business to grow and succeed? Within this there are more specific attributes about the plants that factor into the greenhouse size. The size of the plants need to be taken into account. This is done by knowing how many can be grown per square foot. Let’s take lettuce for example. On average a finished head of lettuce needs 1 square foot when ready for harvest. Taking the amount of lettuce that needs to be grown for the business to make money we can start to get an idea of the area needed.Let’s say you need to produce 1000 heads of lettuce per week. Over the course of a month that is 4000 heads of lettuce, also 4000 square feet of grow space. Is this the square footage that is needed or can we adjust it? Only 1000 heads of lettuce are being harvested per week, this also means you are planting 1000. The germination area and the seedlings need less space per plant before they are placed in their finish spacing. Depending on the variety, lettuce will spend 3-7 days in the germination area then another 5-7 days as seedlings before being spaced out. Each lettuce plant will only spend around 21 days in the 1 square foot finish spacing. By staggering the growing each week and having designated germination and seedling space the needed square footage of grow space has been reduced. You’re planning around the entire growing process and not just the output of the greenhouse.
Height of the plant is also very important to the planned height of the greenhouse. Lettuce can be grown in a greenhouse with shorter walls compared to tomatoes or peppers. Your location also plays a role in the height of the greenhouse. If you are in a more temperate environment the sidewalls can be lower than if you are in the south with higher temperatures. A taller greenhouse will help keep the heat off the plants by giving the air more room to rise above the plants.
The square footage per plant has been worked out but how will they be grown is another factor in the greenhouse space needed. The idea is to get as many plants out of the greenhouse as possible, to be efficient as can be in regards to space. The space for equipment needs to be taken into account. There is the growing system, deep water culture (DWC, NFT etc.) where the plants grow and the auxiliary equipment i.e. irrigation, mixing tanks, fish tanks if growing aquaponically etc. These need space either in the greenhouse or in a head house. Each grow system will have different spatial needs.
In the example of lettuce, space for harvesting will need to be planned out. A lot is not needed but it will still take up square footage. This is usually at the end of the greenhouse where the lettuce will finish growing and be shipped from.
You can really pack the greenhouse and limit the area for planting, harvesting etc. but the impact on labor hours need to be looked at to determine if the tightness of the greenhouse is actually a hindrance to efficiency therefor raising the cost to produce each plant. It’s a planning exercise that needs to be run because the interactions of greenhouse size and labor efficiencies are dependent on each other.There isn’t an average growing space to overall greenhouse size correlation but you will be looking at around 25% of the greenhouse going to none plant growing uses.