Optimal irrigation for perfect blueberries

Since blueberry cultivation requires a lot of water, making every drop count is key to the sector’s survival in a water-stressed country like South Africa. Farmer Wikus Strauss spoke to Lindi Botha about his methods.

Optimal irrigation for perfect blueberries
Through his experiments, farmer Wikus Strauss has determined that blueberry bushes under shade netting don't yield more than those planted in open fields.
Photo: Lindi Botha
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Blueberries are one of the fastest-growing agricultural commodities in South Africa, with farmers capitalising on high market demand and favourable prices.

However, since the expansion of water and soil resources remain stagnant, the key to sustaining the growth of the blueberry sector is to farm smarter, using less water and other resources, for a bigger crop.

With this in mind, Wikus Strauss, co-owner of Seven Growers in Franschhoek, Western Cape, set about establishing the farm’s blueberry production. The farm is partly planted to stone fruit, and the land where strawberries once grew was turned over to blueberries in 2017.

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Strauss aims to increase the area planted to blueberries every year, and is currently at 21ha. He notes that although the market has been volatile, it is beginning to stabilise.

“The past year wasn’t a good one for blueberry prices and sales. There is a lot of volume on the global market, but the quality isn’t always good,” he says.

“Every industry has its ups and downs; I believe there is room for more growth in the [blueberry] industry, and South Africa has a good reputation globally as a quality producer.

Wikus Strauss
Wikus Strauss is the co-owner of Seven Growers farm in Franschhoek, which produces stone fruit and blueberries.

Focusing on quality and taste will be key to growing and maintaining market share.”

The Seven Growers farm is planted to five Berryworld cultivars, and these are being trialled to find the ’holy grail’ of blueberries: those that are big, sweet, of high quality, and can withstand the long shipping period to export markets.

On a farm-by-farm basis
Strauss explains that since every farm has a unique microclimate, what works on one won’t necessarily work on another.

“Each farmer therefore has to trial cultivars to find that which works best for them. We have just removed one cultivar after four years, as it wasn’t performing as it should.

“The general consensus is that the bushes should be able to produce harvests for 12 years before they need to be replaced. But genetics are developing very quickly, and there are always new varieties coming onto the market that have different (sometimes better) qualities,” he says.

“But the cost of orchard development is high, so switching between varieties isn’t always economically viable. There is a lot of trial and error when it comes to finding the right varieties. We have neighbours who struggle with the same varieties that flourish here.”

Strauss’s harvesting season runs from August to December, and he attains a yield of 40t/ha from mature orchards. The standard minimum is 20t/ha, depending on planting density; Seven Growers averages 450 plants/ha.

Strauss is also looking at the differences between bushes planted in an open field, and those planted in pots under shade netting.

Data from the last five years shows that yields don’t differ between the two, although bushes planted in pots do yield far quicker. The cost of erecting nets is, however, reason for pause.

“Nets are a massive investment. It costs about R250 000/ha and can go up to R650 000/ ha, depending on the material you use. Plastic gives you an advantage, as you can harvest while it rains, but the risk is too high for plastic in this area because of our high winds,” he explains.

“There is a slight benefit with the shade netting in that it reduce damage to fruit if there is ice rain around August.

But we have found that if we harvest the first round of fruit before then, the crop isn’t affected.”

The farm has acidic sandy loam soil, which is ideal for blueberry cultivation. The bushes planted in open fields have been placed on ridges to prevent the roots from becoming submerged in water during the rainy season.

The farm receives an average of 800mm of rain per year, which occurs predominantly in winter. This means there is less chance of rain during the harvest season, and thus less chance of damage to the fruit. However, more water is needed for irrigation in the hot summer months.

As blueberries require around 7 000m³ of water per annum, compared with the 4 500m³ for stone fruit, Strauss has had to carefully consider his irrigation system to make the most of every drop of water.

Advanced irrigation
Since there are a lot of river rocks on the Seven Growers farm, Strauss chose to plant a portion of his blueberry plants in pots with a growing medium comprised of coir and perlite.
He notes that the benefit of planting in pots is that the bushes can be moved around the farm if they aren’t happy in a specific microclimate.

“The pH of the soil, and the plants’ water and nutrient uptake, can be far better controlled in the pots, as you can measure the run-off.”

Gutters are placed under the pots to catch any excess water and transfer it away from the roots. They also ensure that any run-off is clearly visible, so irrigation can be managed accordingly.

Strauss explains that irrigation cycles are determined by calculating the field capacity of the pots.

“We measure how long it takes to fill the pots to their maximum capacity of water before the water starts running out of the bottom. We then know that this irrigation time should never be exceeded. There should be slight run-off, but never more than 5%.”

Seven Growers’ pumphouse allows for individual irrigation lines for each block, so they can be managed individually. Seven Growers uses non-leakage valves on Netafim’s driplines, which have been installed at the end of each line to ensure that if the tap in the pumphouse is opened, there is a quick flow of water from one end of the orchard to the other.

Once the taps have been closed, the rate of water withdrawal is also quicker, so the plants at the entry point of the irrigation pipe don’t receive more water than the rest, as they would if the water flowed slowly through the system.

The main irrigation pipes coming out of the pumphouse are split between the rows. They are then split even further into smaller pipes or spikes, with each pot receiving two on either side.

This ensures that should the smaller spikes on one side malfunction, the pot will still receive water from the two on the other side, thereby mitigating risk.

While Strauss maintains that he hasn’t had any issues with the system, he runs peroxide through the pipes once a year as a precautionary measure to clear any blockages and keep the system clean.

Each spike provides 3ℓ of water per hour. They are moved around in the pots twice a year so that the roots don’t have a chance to amass around them.

This also prevents the formation of salt pockets in the pots. Strauss doesn’t follow a set irrigation programme, and instead relies on information from on-farm weather stations, as well as the plants’ actual water consumption rates.

On average, irrigation cycles of 10 minutes each are conducted five times a day. A longer pulse of about 30 minutes is done in the morning, followed by the 10-minute pulses throughout the day, ending with a smaller dose of water in the last pulse, as the plants must not be overly wet during the night.

“Because the pots are first filled to field capacity and then topped up, it means that all of the roots receive an equal volume of water. This ensures that the roots grow evenly and optimally to absorb all the nutrients,“ explains Strauss.

“We can’t confirm this with scientific studies, but it seems as though berry plants grow best with a free-flowing water supply. When the plants need water, it must be there. If the plants wilt, then we stop irrigating, because it means it is too hot and the plants have shut down.“

Quality Management
Given the high summer temperatures in the area, the farm’s irrigation pipes are at risk of heating up to such an extent that the water burns the plants’ roots; water lying idle in pipes have been measured at a temperature of 50°C.

To combat this, Strauss had white irrigation pipes installed, thereby reducing the water temperature. This is especially prudent for newly established orchards, where the bushes aren’t big enough to provide shade for the pipes.

White ’skirts’ have also been placed around the black pots. In summer, they protect the pots from excessive heat, and in winter, they can be lifted to allow the pots to absorb additional heat from the sun.

According to Strauss, there is little data available to determine whether the quality and flavour of blueberries can be manipulated via fertigation. “However, comprehensive studies of strawberries have found that nutrients play a very big role in lifting the Brix level of the fruit.

“Potassium, for example, can lift the quality of fruit. However, potassium uptake is, to a large extent, determined by the plant’s calcium content, and this is very hard to manage. Calcium isn’t easily absorbed by blueberries, so it is tricky to try to lift the sweetness of the fruit through nutrients.”

Strauss believes that, ultimately, the greatest gain in sweetness is determined by genetics.

“You can’t farm fruit into being of good quality if the genetics aren’t there. Nevertheless, good farm management will decide if those genetics will give you a return on your investment.”

Email Wikus Strauss at [email protected].