Magic from the Manual:

Watering Greenhouse Crops

By Terry Ferriss, Ph.D., University of Wisconsin – River Falls

One of the most challenging and sometimes difficult tasks to teach a new employee is how to properly water. Whether it is in the production greenhouse or in the garden center, proper watering is critical to producing and maintaining quality plants and providing quality plants is key for sustained sales. There is a saying in the industry : “the person who controls the hose controls the profits”. Following is a review of good basic watering practices that every certified professional is expected to be able to demonstrate. Further discussion of water quality issues will come in a later article.

Watering Greenhouse Crops

Underwatering

When water is not applied frequently enough, the plants will have a hardened appearance. Underwatering reduces photosynthesis and thus overall growth. In addition, the elongation of young cells is reduced resulting in smaller leaves and shorter stem internodes. In severe cases, wilting, marginal leaf burn, dry patches on the leaves and even death may result.

Near the end of production, growers will sometimes intentionally withhold water to create a temporary, reversible stress on the plant. This is known as hardening-off or toning of the plants. Concurrently with the decrease in water, fertility and temperature may also be reduced to slow the growth rate and develop harder or more toned growth. Hard or toned growth will be able to better tolerate environmental and physical stress during shipping and marketing.

Overwatering

Applying water too frequently will keep the soil pores filled with water and hence reduce oxygen availability to the roots resulting in a weaker root system. The high water content may produce large leaves but they are soft due to the high water content which makes them very susceptible to even short periods of high temperature or high light stress. Overwatered plants wilt rapidly, frequently have excessively long internodes, chlorotic lower leaves, soft growth, and are more susceptible to nutrient deficiencies. It is important to let the media dry down to 40-60% of its original water weight before re-watering. This will allow oxygen to the roots which is required for root development. It takes a lot longer for a plant to recover from overwatering than from underwatering. Therefore if a grower errors in watering, it is better to error on the side of growing on the dry side.

Best Management Practices for Watering

1. Use a well drained media. Proper selection of media will be covered in future articles.

2. Water thoroughly every time the plant is watered. The root zone should be entirely wetted. Historical recommendations include allowing for a 10% leachate with every irrigation to ensure saturation and minimize soluble salt accumulation. As the industry moves towards more sustainable production practices there are many growers experimenting with alternative irrigation techniques to ensure thorough watering while trying to minimize the amount of water run-off.

3. Water just prior to moisture stress. Never wait until the plant wilts. Different plant species will differ in their water requirements. Signs that a plant is approaching water stress include loss of luster of the leaf surface and subtle leaf color changes - typically becoming a bluer or grayer shade of green. The color and weight of the root substrate may also be used to evaluate watering needs. Peat based media will become lighter tan in color as the media dries. Using the "finger test" on potted crops can also be used. Insert your pointer finger into the media and when it is dry to a depth of your middle knuckle it is time to re-water.

4. Allow the plants to dry down between waterings, to 40-60% of it's original water weight before re-watering. Good growers can just lift a pot and sense this weight and know it is time to re-water.

5. Time the watering so the plants are turgid going into the heat of the day, 11 am - 2 pm.

6. Keep the leaves dry to the extent possible. Remember that a free water film on the leaf surface can create an environment conducive to disease development, especially when going into the cooler, darker night-time hours. Watering systems that minimize leaf wetting will help to control diseases. Watering early enough in the day to have the leaves dry when darkness arrives will also minimize disease development.

7. Adjust the frequency of irrigation as other changes in the environment occur including light intensity, temperature, air movement, and humidity. Many of the irrigation systems that are integrated with the computerized environmental control systems will automatically monitor the changing environmental conditions and adjust the watering accordingly. Conditions that increase the plants use and transpiration of water will require the frequency of irrigation to increase, with the reverse also being true. For example, when there are 4-5 consecutive days of cloudy weather the plants may not need to be watered at all, depending upon temperature. Overwatering during cloudy periods can lead to soft growth and even physiological and disease problems. The time interval between irrigations during humid conditions may vary considerably compared to when conditions are hot and dry.

Proper watering is a critical step in producing and maintaining quality, disease free plants. One of the best ways to test your knowledge and ability to water is to try and train someone else to water.

Study Questions:

T F 1. Wetting the leaves when watering greenhouse crops is a good practice

because it helps keep the leaves cool and clean.

2. Which of the following practices would likely result in an overwatered plant.

a. Watering late in the afternoon.

b. Irrigating a potted crop with a volume of water that results in 10% of the water to run out of the bottom of the pot.

c. Water first thing in the morning and late in the afternoon everyday.

d. Never allow the soil to dry out, keep the media wet at all times.

e. Irrigating with a water quality source that has high alkalinity.

3. Good watering/irrigation practices for potted crops include:

a. Syringe the leaves every time to keep the leaves cool

b. Don’t water until the plants just start to wilt.

c. Allow the media to dry down to 40-60% of it’s original weight before re-watering.

d. Time the irrigation so the media is moist during 11 am – 2 pm.

e. Evaluate the color changes in the leaves of the plants and the color changes in the media to help to determine proper time of irrigation.

Answers: 1. False

2. C D

3. C D E

Terry Ferriss is a member of the MNLA Certification Committee and can be reached at terry.l.ferriss@uwrf.edu.