Jun 9, 2020
Blueberry irrigators look for exact combinations

If it’s not wet, there’s not much fruit that you will get.

That could serve as the mantra of Mark Longstroth, Michigan State University (MSU) Extension small fruit educator, who presented tips for effective irrigation of blueberries when he spoke at MSU’s 2020 Pre-Bloom Blueberry Webinar in mid-May.

“Water is pretty indispensable,” Longstroth said. “It is a vital part of the chemistry of photosynthesis. It’s used to transport nutrients up from the roots to the leaves and then to the fruit. It’s vital to growth. All of those plant parts are pumped up by water – that’s how they grow, by pumping up new cells, or the cells that are already there.”

Longstroth said reduced water produces smaller plants, stems, leaves and fruit. “The other thing is, the plant will try to grow more roots,” he said. “As long as it can find water in the soil, it will grow roots where the water is. The leaves pull water out of the soil. That’s how nutrients are taken up in the plant. The roots don’t naturally take up water except under extreme circumstances.”

Related: Four irrigation system options for blueberry fields

The shoots in the plant take off and grow rapidly. Root growth starts before the shoots grow, and once the roots begin to grow, all of the plant’s nutrients are converted into growing new shoots and leaves.

“Root growth drops off and it doesn’t really kick in until later in the season when soil water becomes less available, and that’s when you are sizing fruit in that zone,” he said “So you want to make sure the plant does not run out of water when the surface area of the roots is less, so they become very susceptible to drought symptoms.”

How does the fruit grow for a couple of weeks after bloom when those berries really increase in size? It’s all by cell division – making more and more cells when the fruit is in the active cell division phase.

“And then cell division stops, and after a while we get cell expansion where the cells already there get pumped up by water,” he said.

“If we get off to a good start early, we’re going to have large fruit at the end. If that’s the last cluster in the plant to bloom, they’re never going to get as big as we want to sell into the fresh market. They’ll only be for processing berries, if we can find a market for very small processing berries.”

Pinpointing irrigation needs is a matter of determining:

• How much water is the plant using?
• How much water can the soil hold?
• How much can your irrigation system apply?

“Then we use what we call the checkbox system where we know how much water the soil can hold, and whenever it gets down to a certain capacity, we refill it with irrigation or by tracking the rain.

“I believe that blueberry soil should be recharged whenever the soil water falls to below 50% of capacity,” said Longstroth, who plans to retire from his MSU position at the end of 2020. “By the time half of the water in the soil has been used, it’s time to put more water into the soil for the blueberry plant.”

How much water do Michigan blueberries require?

“About one-half (0.48) an inch for the month of May, which is about 0.12 of an inch a week – just just barely 0.02 inches in a day,” Longstroth said. “That’s when they don’t have any leaves on at all. There’s a huge jump in June where we use almost three inches (2.87), three-quarters (0.72) of an inch a week and one-tenth (0.10) of an inch a day. It’s not like the end of May they start using three inches a month. As those leaves come out, at the end of May we should be using figures that are closer to what June is than to what May is. By the end of June, we’ve got really long days. It’s getting really hot, and we almost double again going into July where we’re using an inch and a quarter a week, about one-fifth of an inch a day for daily use of water.

Graphics: Mark Longstroth/Michigan State University

“In looking at the root zone, the top 1 to 2 feet of soil is what we’re working with,” he said. “We’re working with the roots that the plant has produced in the last three or four weeks. If the soil dries out, and that root production stops, then the plant has a hard time recovering and the root zone won’t start back up.”

Knowing how much water the soil can store is important in managing the irrigation applications.

“Most of our soils (in Michigan) are pretty sandy soils that do not hold much water,” he said. “You don’t want to irrigate past the first two feet of soil where most of your blueberry roots are. Most of them are close to the soil. If you do heavy irrigation, you may be washing most of your nutrients down below the level of the roots. Any water you put on below the root level is actually wasted for you, too, washing away some of your herbicides and things like that, depending on how soluble they are.

“Smaller, more frequent irrigations that do not over fill the soil are better than heavy irrigation cycles that may wash nutrients from the soil,” Longstroth said.

Most of the blueberries in Michigan are grown on sandy loam soil. Some are grown in sandy soils and only hold about an inch of water in the first foot.

“Most of the sandy loams we have will hold between an inch and two inches of water per foot. We’re thinking that 18-24 inches is the root depth, so what we’re working with is perhaps 2-3 inches of water total in the area that we’re at,” he said.

More organic, heavier soils hold much more water than the sandy soils where the water drains quickly. “The loams and the clay can hold on to water really well, but we’re not growing blueberries in those,” he said.

Longstroth measures evapotranspiration with a calculation based on temperature and humidity, that you can be obtained from the MSU Enviroweather website, https://enviroweather.msu.edu/, enabling growers to schedule their irrigation, accordingly.

“You want to irrigate fields whenever you think you’re getting close to the 50% soil capacity so it’s easy for those blueberries to suck water out of the ground and into the leaves to continue to make sugars,” he said. “You want to continually recharge the soil as you approach 50% of soil moisture. If you let them dry out, shoot growth will stop, and fruit growth probably stops before fruit growth.

“If we’ve got 2 inches of water in the soil for 20 days, we should be ready to irrigate again in seven to 10 days,” he said. “In July, we’re going to have to be irrigating every five days, and then we can taper off.

At the start, the plant typically will put on a lot of leaves early on, so the root system in the plant doesn’t need to provide a lot of water to the plant.

“Suddenly, we get a warm, sunny day and the leaves need a lot of water and they wilt because the root system can’t simply pull enough water out of the soil to meet the demand of the shoots, so you’ll see wilting at the shoot tips. There’s a signal right then that tells the shoot tip to stop growing.”

Longstroth said when he sees fields that are poorly irrigated or not irrigated, or the grower has planted into sand producing young plants that put out new leaves, “they look real good, and then they just stop growing for the year because they ran out of water really fast. With blueberries, you’re not going to get any more new leaves from that terminal. It may begin to set fruit buds later, or if you’ve got a pretty good irrigation scheme, it actually may start up and grow again with a second flush of growth later in the season.”

Essentially, the end of growth flush takes place when the terminal bud dies. Shoot growth stops and there are no new leaves. A new terminal bud develops in the axil of the uppermost leaf and buds develop in all leaf axils.

“In May you’ll be irrigating every other week, and by June you will be irrigating once a week,” Longstroth said. “I know many people during the growing season who are irrigating twice a week, or every three or four days.”

Water measurements calculated by Longstroth include:

• Acre inch = 27,156 gallons.
• 0.2 inches = 5,430 gallons.
• 1,244 blueberry plants/acre.
• 4.37 gallons/day.
• If there is one inch of shortage of water, the plant is short 22 gallons of water.
• If there is a 7-inch shortage of water, the plant is short 152 gallons of water.

“When you’re short an inch of water, the plant is short 22 gallons of water that it would have used if that inch had been there,” he said. “In drought years, sometimes we’re short 7 to 9 inches of water. That means your plant is short 150 gallons of water. Imagine how much growth would have taken place if you were able to supply that water.”

— Gary Pullano, managing editor

Photo above: Mark Longstroth, Michigan State University Extension small fruit educator, presented tips for effective irrigation of blueberries when he spoke at MSU’s 2020 Pre-Bloom Blueberry Webinar in mid-May. Photo: Gary Pullano


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