Fruit Trenches

During the 20th century, the Soviet Union was able to cultivate citrus trees (tropical and subtropical frost-intolerant plants) at high latitudes where temperatures dropped as low as -30 C.

Following the Russian revolution, the Soviet Union looked at expanding domestic citrus production, originally concentrated around the Black Sea area, to reduce dependence on foreign imports.

"By 1940, the acreage had grown to 17,000 hectares and production reached 40,000 tonnes, double the annual imports under the old regime. By 1950, the area planted with citrus fruits reached 30,000 hectares (56% mandarin trees, 28% lemon trees, 16% orange trees), and production grew to 200,000 tonnes of fruits per year." [226]

They were able to do this in three ways:

creating cold-resistant citrus varieties through progressive planting
pruning citrus into unconventional shapes
planting citrus in trenches

"The method [of progressive planting] consists of planting a seed of a highly valued tree a bit further north of its original location, and then waiting for it to give seeds. Those seeds are then planted a bit further north, and with the process repeated further, slowly but steadily pushing the citrus variety towards less hospitable climates. Using this method, apricot trees from Rostov could eventually be grown in Mitchurinsk, 650 km further up north, where they developed apricot seeds that were adapted to the local climate. On the other hand, directly planting the seed of the Rostov apricot tree in Mitchurinsk proved unsuccessful." [226]

Citrus were pruned to have compact crowns and grow closer to the ground, protecting the tree from wild temperature swings and cold winds. Trees were planted on terraces to maximize space use in favorable microclimates.

Individual trees are protected during the winter by shelters made of cheesecloth or straw mats, supported by a light frame of poles. Plantations were surrounded by windbreak curtains which channeled cold air masses descending from hills around the plantations. Trees were also planted close together to maximize protection. This system is easier to develop with smaller citrus trees.

Pruning citrus into a creeping form (height < 25 cm) provided even more protection from temperature maxima and minima. In winter, air temperature below the crown was found to be 2.5 - 3 degrees Celsius warmer than air 2 meters above the ground. In the summer, the temperature differential could be up to 20 degrees cooler. Creeping forms also were much better protected from winds, and reduced evaporation in the crowns, reducing water usage.

Creeping citrus trees also produced a higher yield than semi-dwarf varieties, especially during the first years, and fruits ripened earlier.

In order to grow citrus in colder climates with hard freezes, however, they had to grown in trenches.

The depth of the trenches varied between 0.8 to 2 meters in depth depending on how far the ground froze in the winter and the depth of the water table. They were 2.5 m wide at the bottom and 3 m wide at the top for a single row, 3.5 m at the bottom and 4 m at the top for double rows.

Trenches were lined or coated in clay or reinforced brick where necessary. Length of the trenches varied based on the geography but were not longer than 50 m. They were oriented from East to West for maximum sunlight during the winter.

During the winter, the trenches are covered by 2 cm thick wooden boards and single or double straw mats. Boards were sloped at 30-35 degrees. If it got warm enough during the day, the coverings were removed.

This works because citrus plants can tolerate very low light for 3-4 months out of the year. At low temperatures (about 1-4 C) plant metabolism slows.

Glass was used sparingly, as wood is a better insulator and more readily available. Up to a quarter of the trench shelter was made of glass, which were themselves covered in straw mats and a top cover of earth and clay. A few openings would provide light and ventilation.

Creeping citrus in trenches were labor-intensive but simple and cheap and provided high yields.

Other methods of citrus production included transplanting the trees to overwintering sheds, which were sometimes half-buried and heated.

They were also grown in unheated glass greenhouses, especially along the Black Sea coast, known as limonariums.

Citrus were also grown in residences, schools and even in factories and workshops, making use of waste heat.

"Few of these methods would have been profitable under a free trade regime. Considerable research investment went into kickstarting domestic citrus production. Although most methods did not require fossil fuels and were possible using cheap and locally available materials, they were very labour intensive. Domestic citrus production was only possible because it was sheltered – not only from frost, but from foreign competition too." [226]