Highbush blueberries: planting in acidic peat for a real crop

If a highbush blueberry stays green and nearly berryless year after year, the soil is almost always to blame. It needs acidic soil with a pH of 4–5, while ordinary garden soil sits at 6–7. No amount of fertiliser fixes that – the right soil is built at planting time: a wide, shallow hole filled with acidic sphagnum peat, a bark mulch, two or three varieties side by side and rainwater for watering.

Why there are no berries in ordinary soil

The highbush blueberry (Vaccinium corymbosum, kultuurmustikas in Estonian; in our catalogue it sits in the “Blueberry” section) comes from acidic peaty ground. Above pH 5.5 its roots struggle to take up iron and other micronutrients, even when the soil holds plenty. The bush does not die, but it grows reluctantly and barely fruits.

The second quirk is the roots. They are fine, almost without root hairs, and sit in the top 30–40 cm. That is why blueberries tolerate neither drying out nor waterlogging.

How to tell the soil is the problem

• Leaves are pale green or yellowish while the veins stay green – classic chlorosis caused by high pH.
• New growth is short, hardly any strong shoots come from the base, and the few berries are small.

It is easy to check: take soil from the root zone at 10–20 cm, stir it with distilled water roughly 1:2, leave it for half an hour and dip in a pH test strip. Anything above 5.5 means the soil is the problem.

When to plant in Estonia

Container-grown bushes can go in all season, as long as the ground is not frozen. Spring is the safest time – late April and May – with a whole summer ahead for rooting. Autumn planting works too if you are done by early October: the roots need 3–4 weeks in still-warm soil. Freeze–thaw cycles push a late-planted, unrooted root ball out of the ground, so mulch such a planting straight away with 8–10 cm of bark.

Where to plant

Full sun, and a spot where water does not stand in spring. Keep blueberries away from concrete foundations and limestone gravel paths – the lime raises the pH around them.

The planting hole: wide, not deep

• Size: 60–80 cm across, up to 1 m on heavy soil, and 30–40 cm deep. An 80×80×40 cm hole takes about 250 litres of growing medium, so count your bags in advance.
• What to fill it with: unlimed acidic sphagnum peat (pH around 3.5–4.5) or a ready-made blueberry soil. Coarse sand or pine needle litter can be mixed in.
• What to leave out: compost, manure, wood ash, lime, dolomite, black topsoil and multipurpose compost. Standard planting peat for trees and shrubs is already limed and will not do either.

On clay soil

In clay, a peat-filled hole turns into a bathtub: water from the surrounding ground drains into it and sits around the roots, and a layer of sand at the bottom does not help. A raised planting is more reliable: strip the turf, build a ridge of acidic peat 20–30 cm high and about a metre wide, and plant into that.

Planting step by step

1. Stand the pot in a bucket of water for 15–20 minutes, until the bubbles stop.
2. Take the bush out and tease the roots loose around the edge of the root ball; if they are tightly circling, make 3–4 shallow vertical cuts. Otherwise the roots will stay in the shape of the pot.
3. Plant at the same depth as in the pot, or 2–3 cm deeper.
4. Firm the peat lightly around the root ball and water in with a full bucket.
5. Mulch 5–10 cm deep with pine bark, pine needles or conifer sawdust, keeping it off the stems.

Two or three varieties with different seasons

Most varieties are partly self-fertile, but with another variety nearby you get more and bigger berries – yield gains of up to 20% are reported. Plant at least two varieties that flower at the same time, 1–1.5 m apart, with 2–2.5 m between rows. Pick varieties with different seasons and the harvest runs from July into September:

• Early, from the second half of July: ‘Duke’ – one of the earliest; it flowers late and so escapes spring frosts.
• Midseason, late July to August: ‘Patriot’ – large berries, hardy to −35 °C; ‘Bluecrop’ – a proven heavy cropper, hardy to −30 °C; ‘Northland’ – a half-high type about a metre tall, the hardiest of all, for cold and exposed spots.
• Late, from mid-August: ‘Chandler’ – among the largest berries, with a harvest stretching over 4–6 weeks; give it the warmest spot. Very late varieties such as ‘Elliott’ fail to ripen in a cool summer.

Watering, mulch and feeding

• Watering: keep the soil constantly moist. In dry spells a bush needs 25–50 mm of water a week – for a root area of about 1 m² that is 2–5 buckets. Rainwater is best: in Estonia, well and tap water are often hard because of limestone and dolomite, and over the years they push the pH up.
• Mulch: top it up every spring so that it stays 5–10 cm deep. Bark and needles hold moisture and acidity. Bare peat on the surface dries into a crust that sheds water.
• Feeding: nitrogen only in ammonium form – ammonium sulphate or a fertiliser for blueberries and heathers. Feed first as growth starts in spring and again in early June; no nitrogen after the end of June. As a guide, ammonium sulphate at about 30 g per young bush per season, rising to 100–110 g for a mature bush, split into two doses.
• If the pH has crept up: use sulphur – roughly 30 g/m² on sandy soil and 60 g/m² on loam lowers the pH by about half a unit. It takes several months to work.

The first years and winter

For the first 1–2 years it pays to pinch off the flowers in spring, so the bush puts its energy into roots and shoots. Expect a small crop in year three and a full one from year four or five. A mature bush gives 2–5 kg of berries a season.

Mature ‘Duke’, ‘Patriot’, ‘Bluecrop’ and ‘Northland’ bushes overwinter in Estonia without protection. For the first winter, mulch the root area with 8–10 cm of bark or needles.

What not to do

• Do not plant into compost, manure or multipurpose compost – the pH will never drop low enough.
• Do not scatter wood ash or lime under the bush, and do not lay limestone gravel paths next to it.
• Do not feed with calcium nitrate or fertilisers containing potassium chloride.
• Do not hoe deeply under the bush: the roots are right at the surface.

Frequently asked questions

When will I get the first crop?

Flowers appear as early as the following year, but for the first 1–2 years it is better to remove them. A small crop comes in year three, a full one from year four or five after planting.

What should I do with a bush already growing in ordinary soil?

Lift it in spring or early autumn, prepare a wide hole or raised bed of acidic peat and replant it. Young bushes with shallow roots move well, and the chlorosis usually clears within a season or two.

Can I just acidify the soil around the bush?

Sulphur and soil acidifiers help to hold the pH down, but they do not replace an acidic growing medium. Bringing ordinary soil from pH 6.5–7 down to 4.5–5 takes many months, and on clay or limy soil it is next to impossible.

Do blueberries need winter protection?

Mature bushes of hardy varieties are not covered in Estonia. For young bushes in their first winter, 8–10 cm of bark or needle mulch is enough.

From our catalogue

All highbush blueberry varieties are in the Blueberry section, and feeds in fertilisers for berries.