Pleaching or plashing is a technique of interweaving living and dead branches through a hedge for stock control. Trees are planted in lines, the branches are woven together to strengthen and fill any weak spots until the hedge thickens.  Branches in close contact may grow together, due to a natural phenomenon called inosculation, a natural graft. Pleach also means weaving of thin, whippy stems of trees to form a basketry effect. 
Pleaching or plashing (an early synonym) was common in gardens from late medieval times to the early eighteenth century, to create shaded paths, or to create a living fence out of trees or shrubs. It's a craft that had originally been developed by European farmers who used it to make their hedge rows more secure. 
In hedge laying, this technique can be used to improve or renew a quickset hedge to form a thick, impenetrable barrier suitable for enclosing animals. It keeps the lower parts of a hedge thick and dense, and was traditionally done every few years. The stems of hedging plants are slashed through to the centre or more, then bent over and interwoven. The plants rapidly regrow, forming a dense barrier along its entire length.
In garden design, the same technique has produced elaborate structures,  neatly shaded walks and allées. This was not much seen in the American colonies, where a labor-intensive aesthetic has not been a feature of gardening: "Because of the time needed in caring for pleached allées," Donald Wyman noted, "they are but infrequently seen in American gardens, but are frequently observed in Europe."
After the second quarter of the eighteenth century, the technique withdrew to the kitchen garden, and the word dropped out of English usage, until Sir Walter Scott reintroduced it for local colour, in The Fortunes of Nigel (1822). After the middle of the nineteenth century, English landowners were once again planting avenues, often shading the sweeping curves of a drive, but sometimes straight allées of pleached limes, as Rowland Egerton's at Arley Hall, Cheshire, which survive in splendidly controlled form.
In Much Ado About Nothing, Antonio reports (I.ii.8ff) that the Prince and Count Claudio were "walking in a thick pleached alley in my orchard." A modern version of such free-standing pleached fruit trees is sometimes called a "Belgian fence": young fruit trees pruned to four or six wide Y-shaped crotches, in the candelabra-form espalier called a palmette verrier, are planted at close intervals, about two metres apart, and their branches are bound together to makes a diagonal lattice, a regimen of severe seasonal pruning; lashing of young growth to straight sticks and binding the joints repeat the pattern.
Smooth-barked trees such as limewood or linden trees, or hornbeams were most often used in pleaching. A sunken parterre surrounded on three sides by pleached allées of laburnum is a feature of the Queen's Garden, Kew, laid out in 1969 to complement the seventeenth-century Anglo-Dutch architecture of Kew Palace. A pleached hornbeam hedge about three meters high is a feature of the replanted town garden at Rubens House, Antwerp, recreated from Rubens' painting The Walk in the Garden and from seventeenth-century engravings.
In the gardens of André Le Nôtre and his followers, pleaching kept the vistas of straight rides through woodland cleanly bordered. At Studley Royal, Yorkshire, the avenues began to be pleached once again, as an experiment in restoration, in 1972.
Pleaching in art
The word pleach has been used to describe the art form of tree shaping  or one of the techniques of tree shaping.  Pleaching describes the weaving of branches into houses, furniture, ladders and many other 3D art forms. Examples of living pleached structures include Richard Reames's red alder bench and Axel Erlandson's sycamore tower.  There are also conceptual ideas like the Fab Tree Hab. 
- The Complete Guide to Pruning and Training Plants, Joyce and Brickell, 1992, page 106, Simon and Schuster
- Seymour, John (1984 copyright by Dorling Kindersley Limited, London. Text copyright 1984 by John Seymour). The Forgotten Arts A practical guide to traditional skills (in English). page 53: Angus & Robertson Publishers. p. 192. ISBN 0-207-15007-9.
- Oxford English Dictionary
- Mentgen, Glen A. (2000). GROW ON TREES The Complete Guide to Starting Your Own Profitable Tree Farm Includes Production, Maintenance and Marketing. United States of America.: TLC Publishing. p. 120. ISBN 1-929709-03-X.
- The booke of husbandry, John Fitzherbert. London, 1573
- The second book of the English husbandman, Gervase Markham. London , 1614, Part II, ch. VI. Of Plashing of Hedges
- Fischbacher, Thomas (2007), "Botanical Engineering", School of Engineering Sciences @ University of Southampton
- Wyman's Gardening Encyclopedia 1971: "Pleach".
- The Fortunes of Nigel, ch. x, noted by Paul Roberts, 'Sir Walter Scott's Contributions to the English Vocabulary" PMLA 68.1 (March 1953, pp. 189-210) p 196.
- Charles Foster, "The History of the Gardens at Arley Hall, Cheshire" Garden History 24.2 (Winter 1996), pp. 255-271. p 265 and 266:fig 10.
- Eleanor Perenyi, Green Thoughts: A Writer in the Garden (New York) 1981 pp 24-25.
- Quarterly Newsletter (Garden History Society) No. 10 (Summer 1969), pp. 8-10.
- Anne Kendal, "The Garden of Rubens House, Antwerp"Garden History 5.2 (Summer 1977, pp 27-29), p.28.
- Ken Lemmon, "Restoration Work at Studley Royal" Garden History 1.1 (September 1972, pp. 22-23) p. 22.
- Article Title: Art Eco, Photographer Deborah Johansen California Living, SF Sun. Examiner and Chronicle 14 Nov 1980
- Article Title: The Tree Circus, Writer: Fredric Hobbs, San Francisco Sunday Examiner and Chronicle, 23 Nov 1980
- McKee, Kate (2012), "Living sculpture", Sustainable and water wise gardens (Westview: Universal Wellbeing PTY Limited): 70-73
- Article Title: Nature's Home, books, Princeton Architectural Press, July 2005
- Time-Life Encyclopedia of Gardening: Pruning and Grafting