Blackthorn tree (Prunus spinosa)
The blackthorn (Prunus spinosa) is a small native wild plum. Their bitter-sour fruits do not become tender until autumn after the first night frosts – but are still quite acidic even then. Nevertheless, blackthorn is used as wild fruit for the preparation of jam and aromatic liqueur – the so-called “blackthorn fire”.
The spread-area of the Blackthorn tree (Prunus spinosa) extends from Europe to Asia Minor and North-Africa. Like most of our fruit species, the plant belongs to the rose family (Rosaceae) and is classified within the stone fruit family as belonging to the genus Prunus. The species name spinosa gives an indication of the appearance of the shoots, because it means as much as thorny or spined. For this reason sloes are also known in the vernacular as “thorn bush”, “blackthorn” or “blackthorn”. The blackthorn can be found on sunny forest edges, in vineyards and field shrubs from the lowlands to the heights of the Alps.
Blackthorn tree (Prunus spinosa) grow to a height of three to five metres when fully grown and can grow considerably in width with age as they form many root runners. The main branches of the sparse upright shrubs are very strong and densely branched. The bark of twigs and branches is dark brown to almost black. It has a slight sheen and is covered with long thorns.
The approximately three to four centimeter long sloe leaves are elliptically shaped, finely serrated and dull dark green in colour. They are alternately arranged at the shoots and deciduous green. The autumn colouring of the leaves is rather inconspicuous, mostly yellowish to slightly reddish.
The numerous flowers are of particular ornamental value. The buds open from the end of March to April, i.e. before the leaves shoot. The shrubs are then so densely covered with small white flowers that from a distance they look as if they are enveloped in a cloud. The individually standing bowl flowers grow up to 1.5 centimeters in diameter. They exude a light sweet scent and are very strongly flown by insects. If you want to enjoy some flowers on Christmas Eve, cut some branches off on December 4th, Barbara’s Day, and drive them forward in a jug of water in the house. These so-called Barbara branches then bloom a few weeks later.
The flowers develop into edible stone fruit until late autumn, reminiscent of very small spherical plums. They have a thin violet-black skin and are blue ripened. Wild occurrences reach a fruit size of about one centimeter, cultivars can grow up to two centimeters. The flesh is very sour and does not separate from the stone. Due to the high tannin content in the fruit flesh, blackthorn can only be eaten after exposure to frost. The fruits become soft due to the cold and the tannins are broken down. The typical herb-aromatic sloe taste is created, and a particularly rich harvest is reaped especially after long, warm summers. Then yields of up to 30 kilograms per shrub are possible. With a steam juicer you can obtain a deep red juice from which you can prepare a syrup or jelly.
Location and soil
They grow best in sunny warm locations. But they are adaptable and can also cope with shady, cool locations. The shrubs prefer a nutrient-rich and calcareous loamy soil. Blackthorn thrives as a so-called pioneer shrub, but also in very barren, raw and dry locations.
Blackthorn tree (Prunus spinosa) are best planted in autumn. Before planting, the soil should be thoroughly loosened. Subsequently, it is recommended to enrich the soil with a little lime and compost. The planting hole should be so large that the roots fit in well. For bare-rooted shrubs, cut all main roots fresh before planting and remove any kinked or damaged areas. When planting hedges, a distance of about two metres should be maintained, when planting solitaires about three metres. Since the shrubs form numerous root runners, it is advisable to limit the growth rate of the shrubs with a root barrier. Water your blackthorn generously once you have planted the shrub.
They are extremely robust and therefore do not require any special care. Even when dry, the shrubs do not necessarily have to be watered. From time to time you should pull out or cut off the root runners if necessary.
Whether a cut is necessary or not is largely determined by the space available on site and the expected yield. If you want to harvest as much fruit as possible, you should thinn out and rejuvenate the shrubs every three years after flowering by removing the oldest branches. If you don’t care about the fruit and have limited your standing area with a root barrier, you can also let your blackthorn grow undisturbed.
Sloes are used in the wild for afforestation and recultivation measures and are planted as wild hedges and near-natural shrub groups. They are very important bee nourishing and bird protection shrubs. For this reason, sloes are a real enrichment for the natural garden as freely growing hedges for privacy or wind protection. If you want to create a well-kept ornamental garden, however, you will have less pleasure in a sloe, as the shrubs with their dense and somewhat sparse growth look more like foreign bodies here.
Particularly well-known sloe varieties are ‘Nittel’ and ‘Merzig’. They were named after their habitats in the surrounding area of Trier and are characterized by particularly large fruits. They are also excellent for distillate production. The variety ‘Merzig’ grows upright, carries only a few thorns and very small leaves. The shrub grows to a height of about two metres and ‘Nittel’, like ‘Merzig’, grows upright and compact with large, occasionally hanging fruits. Other well-known varieties include ‘Godenhaus’, ‘Purpurea’, ‘Reto’ and ‘Trier’.
Sloes can be multiplied most simply by the numerously formed root runners. One stabs it from the mother plant in the autumn or spring, plants it in another place immediately again and cuts back all shoots by approximately half so that they branch out well.
Diseases and pests
Even if sloes are very easy-care and robust, they are afflicted more often by pests like most native woods. Numerous insects such as the blackthorn moth (Orgyia antiqua), the blackthorn frostbird (Phigalia pilosaria) or the sloe feather ghost (Pterophorus pentadactyla) have their sights set on the bushes. Also the plum louse (Brachycaudus helichrysi), which likes to settle on the related plum, often visits the blackthorn. The use of beneficial organisms or spraying with oleaginous pesticides can keep most of the pests in check if necessary. Birds also contribute to biological plant protection by eating the young larvae of the plant pests. Sloes are also occasionally affected by diseases. These include, for example, shotgun disease and grey mould rot. With young plants, you should remove the infested leaves immediately in order to contain the infections. Sloes are also considered host plants for the so-called Sharka virus. The plants themselves are largely resistant to the virus, but aphids can easily transmit it to nearby, endangered plum varieties.