Blueberries are part of a large genus called Vaccinium, which forms part of the heather family. They are found growing wild across much of North America, from Nova Scotia in Canada down to the south-eastern USA. They are generally divided into 3 different groups:
‘low bush’ – these are the wild forms, still found growing across many areas in the eastern parts of North America. Although commercially grown on a small scale in the USA, you won’t find these offered for sale by nurserymen in the UK.
‘high bush’ – these are the cultivated forms, introduced after many years of intensive breeding and selection, to have heavier crops and larger fruit, but still retaining excellent flavour. Generally, these are taller than their wild counterparts and will reach about 1.2 – 1.5m tall. Most of the varieties you will find for sale are ‘high bush’ – like Earliblue, Herbert or the most common (and still one of the best) Bluecrop. All of these are suitable for UK gardens, and will do well in pots.
‘half-high’ – a fairly recent introduction, these varieties have tried to combine the best qualities of the wild ‘low bush’ and the heavier cropping high bush types. Compact in growth, rarely getting to more than 90cm in height, they have inherited the extreme cold hardiness of their wild forebears, but with bigger fruit. The most commonly found varieties are from the ‘North’ series bred by universities in Michigan and Minnesota – Northland, Northcountry and Northsky. If space is at a premium, these are truly excellent choices.
Blueberries will almost always be supplied as pot-grown plants, whether you buy from your local garden centre or via a mail order specialist. Typically, they will arrive in a 2 or 3 litre pot. It’s a very good idea to pot up into a slightly bigger pot immediately after purchase – blueberries can quickly become pot bound which will inhibit their growth and cropping potential. Whilst it is tempting to immediately use a much larger pot, it is actually best practice to go up in size gradually over the course of a few years. Putting them straight into a much larger pot will leave much of the soil unused and increase the risk of it becoming stagnant.
Blueberries need an acid soil to grow well. The ideal pH is between 4.3 and 4.8, so it is unlikely that your own soil or a general multi-purpose compost will work. Instead, look for ericaceous compost – most garden centres will stock it – which is specifically formulated for all acid-loving plants such as blueberries, azaleas and rhododendrons. Blueberries also like a well-drained, well-aerated soil, so it is a good idea to mix in a little washed sand (it is washed to remove the lime that normal or ‘builders’ sand contains) or composted bark. I tend to use a mix of 60% ericaceous compost, 30% bark and 10% sand. Don’t forget to raise the pots up on crocks or half bricks, especially over winter time, to make sure drainage is good.
Fill the bottom of the pot with enough compost so that the plant is at the right level – don’t plant it any deeper than it arrived. Carefully knock the plant out of its old pot and put in the centre of the new one. Ideally you need a 4-cm gap all the way around the outside, which is sufficient space for you to make sure the compost is firmed in well – a trowel handle is ideal for this. As with all pot plants, leave a gap of a few centimetres at the top for watering. Water in well.
Blueberries need soft water, so you should always use rain water if at all possible. If we have a dry summer and your water butt has run dry, by all means use tap water in an emergency, but revert to rain water as soon as possible.
As plants get older and bigger, it is a good idea to pot up into a larger container every year or two. Blueberries quickly develop a dense, fibrous root system and ideally you want to move them to a larger pot before they get too root bound. The bigger volume of compost will help keep moisture levels even, which means the fruit should swell nicely. Whilst blueberries can survive in dry conditions over the summer, the fruit will tend to stay smaller, the skins will be thicker and they won’t be as sweet. Conversely uneven watering can lead to fruit splitting – lots of water after a dry spell causes the fruit to swell too quickly and the skins can break.
As with initial potting up, just go up one or two pot sizes each time, use a mix of ericaceous compost with some added bark, and firm the compost in well around the edges so there are no air pockets.
Eventually repotting becomes too difficult – pots become too big to move, or are too large for the space available. At this stage switch to an annual top dressing in early March. Carefully knock the plant out of the pot and gently scrape away the top few centimetres of compost, trying not to damage the roots. Pop it back into the pot and use a fresh compost/bark mix to refill to the correct level, mixing in a little slow -release fertiliser with the new compost. This will provide the plant with all the nutrients it needs for the next couple of months, at which point switch to using a liquid tomato feed as the fruit starts to ripen.
Blueberries require relatively little feeding – indeed sudden doses of a fertiliser high in nitrogen, for example using mulches of manure, can do a lot of harm by burning the sensitive roots. If you are potting up every spring and using a good quality compost, the plant should have more than enough food available to it. A very light dusting of a slow-release feed in March is beneficial if your plant is not growing as strongly as you would like, but otherwise I would leave well alone. To encourage good fruiting, I would use a high potash feed (such as a tomato feed) in early summer, but don’t over do it. The best way is to dilute to half the strength it says on the packet, and use this every couple of weeks through April and May. Using a low-strength solution avoids the possibility of root damage, and also means that less is ‘wasted’ – the plant can only take up a certain amount and the rest will just be washed out of the soil by subsequent rain.
Blueberries require very little pruning, certainly for the first few years of their life. If fed and watered correctly, and given an annual top dressing, they will continue to produce new growth and still fruit on old wood. Once a plant has got to about 5 or 6 years old, however, the older growth will start to produce less fruit and you do need to start a programme of renewal pruning. It’s actually very simple – some time in late winter (February to mid-March is ideal) select a piece of the oldest, thickest wood and prune back to few centimetres above soil level. This will open up the plant to allow better air circulation and will provoke dormant buds at the base of the plant to send up healthy new