The middle of winter may seem too early to be thinking about your irrigation program for next season, but it’s really not that early to start the planning process. Having been in the consulting business for more than 20 years, specializing in irrigation scheduling for winegrapes, I have found that there are three different grower perspectives on this topic.
You can best determine a grower’s attitude by asking, “Are you satisfied with the way you decide when and how much to irrigate?” or “Are you at your maximum potential crop yield and quality level?” How would you answer these questions? If your answer is:
• “Yes, every year I’m satisfied with my production and I don’t need to change anything” — stop reading if you are “that confident.”
• “Yes, most years I seem to produce an average crop (compared to my neighbors)” — stop reading if you are satisfied with “average in most years.”
• “No, while I am usually above average in production and quality, I can see room for improvement” — keep reading, you are a person that I want to talk to.
Grape Growing Objectives
Is your goal to produce the maximum yield possible or is it to produce fruit with certain quality characteristics sacrificing some crop yield in the process? This is a very important question to answer as you decide how you are going to make irrigation decisions, and further, what you expect the vines to look like during the season.
With a production goal in mind, you must also have approaches in place that allow you to monitor the growth of the vines and the soil moisture content. As you move through the season, you will gain an understanding of the relationship between vine growth and soil moisture levels. With this understanding, you will find that you can manipulate the soil moisture levels (through irrigation) and produce the vine growth that meets your objectives.
If you are a grape grower without a supplemental irrigation system and count on “Mother Nature to provide” (dry farming), you should consider whether or not you are meeting your production goals and how supplemental irrigation might help.
Monitoring Vine Growth
A grapevine becomes stressed when the soils have dried out to a point where the vine cannot acquire enough moisture to keep up with the climatic demands of the day (the evapotranspiration demand). If you are not irrigating enough to meet the daily needs of the vines, one of the first signs is the shortening in length of the new internode growth of the canes.
As the soil becomes dryer, the tendrils on the basal portion of the cane dry up and the overall cane length is obviously shortened. As the soils become even dryer, the cane tendrils dry up at mid-cane, then closer to the growing tip and eventually the growing tip turns from green to yellow to brown and falls off. At this point, the vine is under a very significant level of stress. With well-timed irrigations, you can stop and hold vine stress at any point in the process.
If you are after maximum production it means that you do not want any appreciable stress on the vines during the growing season. You can expect some normal shortening of the new-growth internode lengths, but only when it occurs naturally at mid and late season.
If your objective is to manipulate the quality characteristics of the grapes, then you will manipulate the soil moisture by limiting irrigation amounts so that the vine will move into a stressful condition at certain times of the season. The times of the season that you want to stress the vines will be a function of the fruit quality characteristics you want to achieve.
Monitoring Soil Moisture
Irrigation decisions become much easier if you are measuring the soil moisture content and have gained an understanding of the effect of soil dryness on vine growth characteristics. Therefore, if you are serious, you should be using soil moisture measuring instruments, using a hand soil probe, and/or hiring a consulting service to provide this information.
If you are going to “do-it-yourself,” selecting an instrument to measure the soil moisture is the next step. Instrument selection should be based on two things: the range of soil moisture levels that you expect in your vineyard, and the frequency of data collection that you desire. Certain instruments work best in moist soil conditions (for maximum production goals) while others operate best in semi-dry and dry soil conditions (maximizing fruit quality characteristics).
Installing instruments that work well in moist soils and reading their values once per week is usually sufficient for a “maximum production” goal. If you want to manage more intensively (manipulating fruit characteristics), then you can install a data logger that will record soil moisture levels every day or every six hours. You then go to the logger once a week, download the data, and view it on your computer. If you just can’t wait for your data, then you need a field monitoring station which uses a cell phone to upload soil moisture data as often as once per hour. The capital cost of these various approaches ranges from $500 to $2,500 per vineyard location with an annual operating cost of $50 to $500 per year.
The bottom line is, decide on your grape growing objectives for next season and how you are going to achieve them. If you don’t make these decisions, then, as the saying goes, “If you don’t know where you are going, any road will get you there.”