The title says it all. Plants need it. In the greenhouse, you are the only source to supply it, since the greenhouse cover very effectively keeps out the rain.
Why so important? Water is the largest component of cells, the building blocks of plants, and even animals. Plant cells contain about 90% water by weight. With all of this water, plants can be simplistically described as tubes of water. The movement of water through these “tubes” carries food and minerals throughout the plant.
However, if the water content decreases, just a little, leaves begin to wilt, losing their normal turgid shape. And if they wilt even further, these leaves may never recover, and other plant parts such as flowers will be damaged as well.
Despite the overbearing need for water in plants, they are not that efficient at holding onto it. Leaves are filled with tiny holes, known as stomata, that literally leak water vapor into the air. This constant loss, known as transpiration, increases with higher temperature, higher wind speed, and lower humidity. So, hot, dry, windy conditions dry out plants the most, increasing their water demand.
While some specialized desert plants, known as xerophytes, have special adaptations to reduce water loss, the kinds of plants we grow for food in the greenhouse and field are very poorly designed for water retention. Cucumbers, for example, have more than 60,000 stomata per square centimeter.
The total surface area of stomata makes up between 1% and 3% of the leaf surface of most plants, allowing the free loss of water to the surrounding air. That is why we have to have a ready supply of water, via an irrigation system, to deliver water to growing plants.
Some Water Recommendations
1) Before starting a greenhouse vegetable business, get your water tested. Do not assume the water is good quality because it is from a well or any other source. A good water test will help you decide if you might need another source.
2) Plan on having a good water supply. Estimate water use as 3 quarts of water per day per plant. Using this high estimate, your greenhouse with 500 tomato plants might use 375 gallons per day. Although it will use much less when plants are still immature, plan for maximum needs.
3) Design the floor for water drainage. With 300 to 400 gallons of water applied per day, there will be a lot of water draining through containers, and it needs to go somewhere. Puddling in the greenhouse is almost a sure way to promote diseases that can kill your plants. Water either needs to percolate through the floor or drain to one end of the greenhouse before it exits. A 1% to 2% slope is recommended.
4) Check the pH of your source water, but also check it after adding fertilizers. The pH of the nutrient solution will not be the same as the pH of your source water. If too high or too low, you may need to adjust it. Acids, such as sulfuric, phosphoric, and nitric, will lower pH. To raise pH, use potassium carbonate or bicarbonate, or potassium hydroxide. For tomatoes, your target range is 5.6 to 5.8, or close to it. If it’s within a couple of decimal points of this range, it is probably best to leave it alone.
5) Don’t let plants wilt. Wilting can cause poor pollination leading to small fruit or low fruit set. It also can keep nutrients from reaching all parts of the plant, leading to deficiency symptoms, lower fruit quality, and lower yields. Blossom end rot, for example, is very common in crops that wilt, even for just a few hours. Without a steady water flow, calcium can’t make it into the developing fruits.