only when used in combination; shoots that were only girdled or only treated with IBA did not root.

Plant shoots used in layering are usually etiolated, due to the exclusion of light from the shoots as they grow through the medium covering them. Etiolation enhances rooting by causing a number of physiological and anatomical changes in the plant's stem, including increasing carbohydrate accumulation in the etiolated area, increasing the number of parenchyma cells above the girdle, reducing sclerenchyma cell formation, reducing lignin production, and increasing the stem's sensitivity to auxin (Hartmann et al., 1997). Etiolation in combination with wounding or girdling can be especially effective in promoting rooting in difficult-to-root species.

A study examining the rooting of apple (Bramley's seedling) cuttings found that only etiolated cuttings produced roots, and that girdling of the etiolated stem further increased adventitious root formation (Delargy and Wright, 1978). The effect of girdling in this study was secondary to the effect of etiolation, as all etiolated treatments produced roots, even un-girdled cuttings, whereas girdling without etiolation did not enhance rooting. A later study by Delargy and Wright (1979) found that auxin application to etiolated cuttings enhanced rooting. Auxin application to unetiolated cuttings resulted in very low rooting percentages, whereas auxin applied to etiolated cuttings resulted in 76% rooting. Studies of cutting propagation of Carpinus betulus found that rooting increased in etiolated cuttings taken from plants which had been subjected to light exclusion (Maynard and Bassuk, 1992). Cuttings taken from shaded stock plants were also more responsive to auxin application. Another study by Biricolti et al. (1994) investigating rooting of chestnut in a stooling system determined that the effects of girdling and etiolation were found to have an interaction, and thus the effects of each could not be separated, suggesting a synergistic effect on adventitious rooting in chestnut.

Trench layering is a little-utilized method of propagation in which mother plants are planted in a horizontal position and covered with soil, sawdust, or some other type of rooting medium. One advantage of trench layering is that with a larger number of buds remaining as the plant is laid horizontally, one would have a larger number of shoots to work with in a smaller area. A study by Suriyapananont (1990) comparing air and trench layering in apple rootstocks found that trench layering was a superior method for some