How deep should bulbs be planted? According to a study published in the Journal of the American Society for Horticultural Science it may not matter because bulbs can move themselves deeper into the ground, seemingly in search of better, moister growing conditions.
According to Dr. A. Carl Leopold of Cornell University, bulbs such as tulips and lilies respond to being planted too shallowly literally "pulling" themselves deeper into the ground. He says that:
"One doesn't think of plants moving, and especially moving down into the ground, but our research proved that this movement occurs."
Carl Leopold and his colleague, the late Dr. Modecai Jaffe, had worked on the physiology of "contractile roots" - roots that are responsible for bulbs' movement. He explained:
"Negative growth is very rare in plants, and the sort of contractile proteins that are so well known to drive contraction in animal muscles do not occur in plants. We selected this work as a divergence from the usual studies of growth, and introduced the idea of contraction."
Dr Leopold added that none of the hundreds of books written about plant root growth mentioned the phenomenon of negative growth.
The study was focused on a variety of Easter lily - the "Nelly White". it was found that Contractile roots responded to light signals perceived by the bulb with some types of blue light forcing the formation of new contractile on the bulbs and helping initiate remarkable bulb movement.
Carl Leopold explained that "contraction is evidenced by a formation of epidermal wrinkles, starting at the base of the root and advancing toward the root tip. The movement function occurs in shallowly planted materials, is lessened at deeper locations, and ceases at a vermiculite depth of 15 centimeters. Movement of the bulb in the soil is achieved by a hydraulic shift in cortical cells. Root contraction is stimulated by light."
The research team found that the light stimulus was 'perceived' in the bulblet or the subtending leaf and that light responsiveness lessened as roots aged. Experimenting with light of different wavelengths they also found that the contraction response was triggered most often by blue light. As blue light most effectively stimulated movement they suggest that there was a blue-absorbing pigment in the lily bulbs. The contraction signal was transmitted from the bulbs down into the roots which did not themselves respond to light. The proportion of contractile roots formed decreased with the depth of bulb planting.
Carl Leopold says that many plants can move down into the soil to achieve greater stability or, as with bulbs and tubers, to find better environmental conditions. He concluded:
"I have some lily bulbs that were in the ground for nearly a decade, and I was astonished to find the bulbs moved themselves over a foot into the ground!".
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