What are chill hours?

Blueberry plants rest over winter. To bud properly in spring and flower abundantly, they need a certain number of hours below 7 °C. This figure is called the chilling requirement and is given in chill hours. In most of Central Europe the needs of Northern Highbush varieties are usually met without trouble. Spring frosts during flowering deserve more attention.

‘Clarksville’ ripens 10 to 14 days earlier than ‘Duke’, one of the best-known early varieties. You can pick blueberries from it right at the start of the season, when demand for fresh fruit is highest. The berries are medium-sized, a lighter blue with a waxy bloom, reasonably firm and distinctly aromatic. Interestingly, their flavour keeps developing and improving during cold storage.

It needs only 400 to 500 chill hours, which is relatively little for a northern variety. It therefore suits milder winters or tunnel growing, while still keeping strong frost hardiness. It also has good natural resistance to common blueberry diseases.

Strengths: ultra-early harvest, aromatic flavour that improves in storage, low chill requirement combined with strong frost hardiness, reliable yields, good disease resistance.

Watch out for: the berries are medium-sized and do not reach the size of later varieties. Growth is semi-upright to spreading, so it needs shaping pruning to stop the branches bending to the ground under the weight of the crop.

‘Miss Alice Mae’ (syn. ‘TH-921’) is an early variety bred in Georgia, USA. It ripens at about the same time as the established early variety ‘Star’, but the fruit develops faster, in 60 to 68 days. The berries are medium to large (16 to 19 mm in diameter, 1.6 to 2.0 g) and purple-blue with a waxy bloom. The flavour is very good, the texture smooth and the sugar content (°Brix) higher than in ‘Star’ and ‘Camellia’. After picking, a small stem scar remains with minimal tearing, and the berries store well.

The bush is vigorous and semi-upright: at four years old it reaches 1.3 to 1.6 m, and 1 to 3 main shoots usually grow from a narrow base. Yield is very good: 4.5 to 6.8 kg per plant on plants four years and older under optimal conditions. It needs about 500 to 550 chill hours.

Strengths: early ripening, firm and sweet berries with good storage life, high yield and vigorous growth, low chill requirement.

Watch out for: frost hardiness is lower, as is typical for southern varieties, so it is not suited to cold northern or higher-altitude areas. It flowers early. It is only moderately self-fertile, so plant another variety next to it for a better crop.

‘Titanium’ (syn. ‘OBF0622’) is a modern variety bred in Oregon. There it reaches 50% ripeness in the first week of July. The berries are medium to large (about 18 mm), slightly flattened and round, purple-blue with a waxy bloom. Its main strength is extreme firmness: the berries resist bruising well, the stem scar left after picking is very small (about 1 mm) and dry, and storage life is excellent. The fruit contains very few seeds (often just 0 to 7) and the flavour is a harmonious sweet-tart, sweeter than ‘Draper’.

The bush is vigorous, upright and rather narrow: at 3 years old it is about 106 cm tall and 89 cm wide. Yield is medium-high to high. It needs about 1000 chill hours, so it is meant for mild and cooler climates.

Strengths: extremely firm berries and a dry scar, very few seeds, sweeter flavour, upright growth that takes up less space, medium-high to high yield.

Watch out for: the chill requirement is high, so it is not suited to very warm areas without frosty winters. The narrow, upright habit may call for pruning that lightly thins the bush and opens up its centre.

‘MegasBlue’ (syn. ‘OBF0604’) was bred in Oregon as an improvement on ‘Draper’: it has a more vigorous bush, a higher yield and larger berries. In Oregon it reaches 50% ripeness in mid-July. The berries are very large, up to 20 mm, shaped like a flattened sphere, with a powdery purple-blue colour and a pronounced waxy bloom. They are firm, and the stem scar is small (1.2 mm) and dry. They contain 0 to 20 tiny seeds, and the flavour is good, moderately sweet with low acidity.

Yield is exceptionally high and the berries ripen in a concentrated way, meaning most of the crop ripens within a short time. The bush is upright but very dense and broadly spreading: at 3 years old it is about 104 cm tall and up to 170 cm wide. Keep this in mind when planning the distance between plants. It needs about 1000 chill hours.

Strengths: very large, attractive berries, exceptionally high yield, concentrated ripening (the crop is picked in a shorter time), firm berries with a small dry scar, vigorous growth.

Watch out for: the wide, dense bush needs more space and careful thinning pruning so that air and light reach the centre. The chill requirement is high. If you want a longer picking period, combine it with varieties that ripen at other times.

‘Fayetteville’ comes from the University of Arkansas breeding programme and was developed with an emphasis on outstanding flavour. It ripens mid-late, so it extends the season at a time when early varieties are finished. The berries are medium-sized (15 to 17 mm, 1.6 g on average), blue with a good waxy bloom and reasonably firm, so they keep well after picking. Flavour is its trump card: the berries are exceptionally aromatic and distinctly sweet, with a sugar content above 11 °Brix.

The variety has strong frost hardiness and performs steadily even in cooler and higher-altitude areas. It needs only 400 to 500 chill hours. Growth is semi-upright to spreading. The plants set fruit well and give reliable yields.

Strengths: exceptionally sweet and aromatic flavour, mid-late harvest, strong frost hardiness, good berry firmness, reliable yield, low chill requirement.

Watch out for: the berries are medium-sized and do not reach giant dimensions, though their flavour more than makes up for it. The spreading habit calls for shaping and thinning pruning so the branches do not bend to the ground under the weight of the crop.

Soil and watering for all varieties

All highbush blueberries need acidic (pH 4.5–5.5), light, peaty and well-drained soil with regular watering. This applies to all five new varieties.

Varieties protected by breeders’ rights

All five varieties are protected by Plant Breeders’ Rights (PBR). Commercial propagation of these varieties without a licence is prohibited.