Greenhouses over Winter

How To
Written by How To on
Greenhouses over Winter

The greenhouses shown in the Grow-Box Gardening book are not the same as Jacob has recommended during the past 20 years – at least for most gardeners. The quanset or ‘Tube’ type greenhouses don’t have good ventilation, and some aren’t strong enough to hold up to severe weather. They are ok for small gardens, however, and cost less than the “continuous ventilator” type that we recommend now.

The Grow-Box Gardening greenhouses also will not grow nearly as many vegetables as the ones we use today. For example, a 30′-long X 10′- wide X 8′-high tunnel style greenhouse will grow 3 20′-long rows of tomatoes, with 31 plants per row – growing them 8″ apart, for 93 plants total.

To grow 500 tomato plants, with proper room to grow to maturity, will require a greenhouse 20′-wide by 75′-long. The most efficient way to grow would be to have 2 sets of 30′-long beds with 5′ aisles on the ends and in the center.

Single rows of plants, in 18″-wide beds, would be planted on both outside walls. Aisles of 3′-width would then separate two sets of 4′-wide boxes. The 4′-wide boxes would hold 2 rows of plants. In the 18″ boxes the plants should be planted on the side of the box nearest the greenhouse wall. In the 4′ boxes the plants should be planted 10″ in from the outside walls. This gives 6 rows of plants in a 20′-wide greenhouse.

Planting 8″ apart in 30′-long boxes will allow for 46 plants per row. In a 75′-long greenhouse you will have 12 rows with 46 plants per row, or 552 plants.


Does this seem like a lot of effort, time, and expense to go through for only 550 plants? Let’s consider the costs and possible rewards.

The greenhouse should be constructed of 8 or 10-mil dual-wall polycarbonate – or at least two sheets of greenhouse plastic placed inside and outside of the studs. With dual-wall polycarbonate costing over $1 per square foot, that will cost $3,000+ ((75′ X 32′ 10′ X 32′) X $1.20), whereas 6-mil greenhouse plastic will cost closer to $300+, so you must decide which way you want to go based on longevity, heating costs, appearance, etc. In either case the greenhouse and watering system should cost much less than $10,000.

Heating costs vary greatly, depending on the type of heating, average temperatures, location in the country/world, etc.

Now lets consider the possible rewards. And keep in mind that a really good hydroponic grower (with over $1,000,000 per acre in capital investment!) can grow 660,000 in one acre. The greenhouse we describe is 1/29th of an acre, so to match the best hydroponic growers we’d need to produce more than 22,000 – and 1/30th of $1 million is $33,333 in capital costs – just to keep things in perspective.

Assuming you start planting by December 1, if you are able to keep your greenhouse heated to 75+ degrees fahrenheit during the day, and more than 60 degrees at night, you should expect 4 months from seeding to first harvest, and you can expect 8 months of harvest after that.

Each healthy tomato plant, if using Big Beef or Better Boy, would be expected to produce 20++# of ripe fruit. With 550 plants this would be 11,000+# – or about 1/2 what the best hydroponic growers produce.

And finally, what will this crop be worth? Again there are MANY variables, but if you are to market in April you will have NO local competition, and should be able to get a wholesale price of $1 or more per pound. From mid-July to the end of September the prices will be about 1/2 that, and then will go up again as most local sources quit producing. If you could sell 2/3 of your crop for $1 and 1/3rd for $.50 you would yield about $9,200.

Now, you’re saying that’s not much! But it’s also not a big operation. Suppose you were to expand to just 1/2 an acre and build 12 greenhouses. You could earn $100,000 per year, every year, with less than that as total capital required. That is MUCH better than the hydroponic growers do, believe me.

If this interests you, I advise you to start small. Learn with one greenhouse, then invest your earnings and grow. With only $7,500 and your own efforts, along with utilities, you could be at the 12 greenhouse level of growing in less than 4 years!