Success with Blueberries

We have had blueberries in our garden for many years, but in the last few years they haven't done so well, so I decided try and find out why, learn from those whose blueberries have been an ongoing success, and start a brand-new blueberry patch. Here is what I learnt.

Key facts about blueberries

• Soil - blueberries require fertile well-draining soil and can only optimally access the nutrients in fertile soil if the pH of that soil is between 4.5 and 5.5. That is significantly lower than the pH in most Tasmanian food gardens.

• Mulch and water - blueberries root shallowly and are not drought-tolerant, so it is important to make sure that soils are moist and mulched at all times.

• Pollination - blueberries are 100% insect-pollinated. Bees are the most significant pollinators. It is best to plant two or more varieties that flower at the same time in close proximity, as this results in more berries of larger size.

• Blueberry varieties – Some blueberry varieties are deciduous. Others don’t lose their leaves in winter. Some blueberry varieties do best in Tasmania’s cool climate. Other varieties are more suited to warmer climates. The blueberry varieties that are commonly grown in Tasmania can, if cared for well, take 7 years to reach full production and remain productive for up to 30 years.

• Benefits – blueberries contain excellent amounts of vitamin C and K. They are one of the best natural sources of antioxidants. They help lower blood pressure, prevent heart disease, improve memory and help in exercise recovery.

We will look at these points in more detail below.

In the wild

If you want to know what a plant’s optimum growing conditions are, consider the climate and soil where the plant originated in the wild. Modern bred and cultivated varieties often have characteristics that go all the way back to where the plant came from in nature.

I came across a very interesting fact sheet by the Tasmanian Institute of Agriculture with the title Mulch affects the growth of newly planted blueberries (2009). The fact sheet discussed how hardwood sawdust (aged for 10 years) was applied to soil to a depth of 10 centimetres. Young blueberry plants were planted in this soil and mulched with the same sawdust. 

Testing showed that the blueberry plants grown in this sawdust environment were significantly taller and had significantly more stems. Root development was much broader and extensive than plants that were not subjected to this treatment.

The researchers had re-created the conditions that naturally occur in forests where wild blueberries grow.

Blueberry varieties

There are many blueberry varieties, but they are not all suited to Tasmania’s climate. Blueberry plants only produce berries if they get chilled in winter. Different varieties have different ‘chill requirements’, that is the number of hours over winter when the temperature is below 7 degrees Celsius (see ref. 3 at the end of this blog post).

Most blueberry varieties are deciduous and create a nice show of coloured leaves in autumn. There are other varieties that don’t lose their leaves in winter.

 

Northern Highbush varieties need the most chilling and are the most common varieties in Tasmania. They are deciduous and tolerant of frost, but Spring frost can kill open flowers, so please cover your bushes with hessian on frosty nights. Some Northern Highbush blueberry varieties can live up to 30 years. 

Northern Highbush varieties commonly available in Tasmania are Denise (up to 1.5 metres high), Brigitta (up to 2.4 metres high), Bluecrop (up to 2.1 metres high), Northland (up to 2.1 metres high) and Blue Rose (up to 2.1 metres high).

One blueberry bush on its own or several bushes of the same variety will do fine, but it is generally recommended that you plant together two or more varieties that flower over the same period of time, because that will increase your crop, both in size and quantity of berries.

Bluecrop and Brigitta are often mentioned as varieties that go well together.

Blue Rose, Denise and Northland are also varieties that cross-pollinate well.

It is generally recommended that you put at least 1.2 metres between plants, because they can become quite big if they take off.

 

For a detailed description of many of the blueberry varieties available in Australia see Blueberry Varieties by Berries Australia here: https://berries.net.au/home/about/blueberries/varieties/

Another good source of info on blueberry varieties is https://www.daleysfruit.com.au/ 

If you buy blueberry plants at your trusted local nursery instead of a nationwide hardware store you won’t have to worry about getting an unsuitable variety because they will only sell varieties that have a proven track record in our Tasmanian climate.

 

Soil

Blueberries require well-draining soil with good levels of organic matter and a pH of less than 5.5, ideally in a spot with all day sun. 

The soil in my old blueberry bed was not well-draining at all. It clay-content was far too high and over the years we had compacted that by walking around bushes while picking berries. We won’t be stepping onto our new blueberry bed.

The pH of our old blueberry bed was also a problem. I fed my blueberries every Spring, but the plants had difficulty accessing the nutrients because the pH of my soil was around 6.9. That is a great pH for most crops, but completely wrong for blueberries. Many Tasmanian food gardens would have this problem.

Please test the soil close to the stems of your blueberry bushes (find out how in blog post Acid or Alkaline on the Food Garden Group blog). The pH needs to be between 4.5 and 5.5.

So what options did I have in my food garden where everywhere the pH is around 6.9? I had two choices:

1. Bring down the pH of one particular garden bed from 6.9 to below 5.5 and then plant new blueberry bushes there
2. Replace the soil in that bed with purchased soil that already has a pH below 5.5

After reading a lot on the subject I concluded that ….

• Reducing the pH of my organically rich food garden soil from 6.9 to let’s say 4.5, that is reduce the pH by about 2.5 points, would be such a radical change, that it would kill most of the soil organisms that at the moment happily live in soil with pH 6.9, and thus make the whole exercise pointless.

• Therefore, replacing the soil in my new blueberry bed with new soil that already has a low pH was the only option.

Just by coincidence we had just bought bags of soil for a few other garden projects. Here is what I found when I measured their pH:

• Osmocote Soil for Roses, Gardenias and Azaleas – pH 5.75
• Debco Rose, Gardenia & Azalea soil – pH 6.25
• Osmocote African Violet & Cyclamen Mix – pH 5.25

Please realise that these outcomes were for just a limited sample of bags. Other bags of the same products may give slightly different results, and there may be other products out there that give an even more blueberry-friendly pH.

If you would want a lot of soil with a low pH, it could work out much cheaper to visit your local soil suppliers, test the pH of soil they recommend, and buy that if its pH is between 4.5 and 5.5.

In my case, after considering the price of each product, I decided to dig a big hole for each new blueberry bush and fill each hole with the first product in the list above.

But before filling the holes I will mix something into that new soil that will hopefully lower the pH a bit further still.

Elemental sulphur is an organically allowed product that is broken down by soil microbes over a period of time into sulphate. The sulphate then lowers the pH. 

You too could use elemental sulphur to lower the pH of your blueberry soil. Please do so in Spring when the microbes in your soil have woken up from their winter sleep and will begin to work their magic. 

The world of organic growing is not always one of instant gratification. It can take from half a year to a year (yes, you are reading this correctly) before the pH of your soil is measurably lower. 

There are ready-to-use sulphates for sale that do the job almost instantly, but these substances are expensive and they poison your soil. I am not being too precious here. Aluminium (from aluminium sulphate) kills micro organisms and is not something you want in your food.

I found many other suggestions for lowering the pH of blueberry soil, some incorrect, others not very organic:

• Pine needles make good mulch for blueberries, but will only lower the pH if they are green. The pH of brown pine needles is almost neutral (see for instance article Do pine trees and pine needles make soil more acidic? - University of New Hampshire).

• Peat moss does bring down pH effectively, but the harvesting of peat moss totally destroys the natural environment where peat moss is found, so I recommend that you don’t use it.

• Coffee grinds only have a slight effect on pH as the pH of spent coffee grinds is around 6.3.

• Vinegar lowers pH very effectively, but only for a short time, and kills most soil life that it comes into contact with.

Woven planter bags are a good alternative to growing blueberries in the ground. FGG member Mandy is very happy with her large strong North GCP 45 litre SQUAT plant bags that provide good drainage and that are advertised as ideal for blueberries. They are readily available in many places, for instance at: https://planterbags.com.au/product/45ltr-squat-woven-bag/ . 

The Denise variety of blueberries is ideal for planter bags as they are somewhat smaller plants with a more compact root system.

Mulch and water

Blueberries root shallowly and are not drought-tolerant, so it is important to make sure that soils are moist and mulched heavily at all times. Regular drip irrigation is a good way to achieve good irrigation.

Pine needles make a great mulch for blueberries, although only fresh green pine needles will help to lower the soil pH.

Earlier in this blog post we saw how 10 years+ aged old hardwood saw dust is an ideal mulch, but I have no idea where you would find any. 

Please don’t be tempted to use fresh sawdust or fresh woodchips to try to lower pH. They may contain naturally occurring tannins that can kill or damage your plants. Sawdust and wood chips take a long time to decompose and in so doing consume a lot of nitrogen from the soil. There may be a compaction problem with not-completely-aged sawdust, so it might actually keep water out. Sawdust or woodchips from treated pine should be avoided because they contain non-organic chemicals.

It might also be good to know that ….

For many berry varieties winter is the best time to buy plants. Not for blueberries! Buy them in Spring, when deciduous varieties have young new leaves and nurseries have received new stock.

Blueberries are poor competitors against other plant roots, so plant them in an area where there are no competing roots from nearby trees or bushes and make sure that weeds don’t establish themselves in their root-zone.

If you can bring yourself to do it, remove flowers on your young blueberry plants in the first few years, so plants focus on growth.

Blueberries love to be fed blood and bone and well-rotted compost in early Spring. Make sure that whatever you add does not contain lime or dolomite.

Berries develop in clusters (as you can see below) and don’t all ripen at the same time (as you can see below).  A blueberry is ripe if it easily separates from the plant. If you have to pull, the blueberry is not quite ripe.

Pruning blueberries is covered in Food Garden Group blog post Pruning Berries.



May your blueberry crops be plentiful .....

Max Bee
Food Garden Group coordinator

References

Some of the info used in this blog post came from:

(1) Tasmanian Institute of Agriculture Fact Sheet Mulch affects the growth of newly planted blueberries: https://www.utas.edu.au/__data/assets/pdf_file/0004/1662250/Fact-Sheet-Blueberry-Establishment-Fact-sheet-_2007.pdf

(2) Tasmanian Institute of Agriculture Fact Sheet Blueberry Growing in Tasmania:
https://nre.tas.gov.au/Documents/WfW-Blueberry-factsheet.pdf

(3) Blueberry Varieties by Berries Australia here:
https://berries.net.au/home/about/blueberries/varieties/

 

(4) Do pine trees and pine needles make soil more acidic? by University of New Hampshire:
https://extension.unh.edu/blog/2019/10/do-pine-trees-pine-needles-make-soil-more-acidic