Tree shelters were originally developed to prevent biotic stress caused by mammalian herbivory. This was accomplished by enclosing the seedling in a protective tube open at both ends to allow root and shoot growth. The height of the tube was a function of the stature of the herbivore. For example, 600-mm is considered adequate protection against rabbit browsing while 1200- to 1500-mm height tubes are generally prescribed to prevent deer browsing.
It was also noticed that young trees grew faster in tree shelters. The mechanisms behind this growth increase were not well understood. Tree shelters (aka grow tubes) were adapted for use in grape establishment and had the added benefit of reducing and accelerating stem training.
Accordingly, a function of a grow tube is to facilitate and promote rapid and permanent plant establishment, even where mammalian herbivory is not a factor. In horticultural systems plant survival is generally not at risk, so the purpose of a grow tube is to hasten and lower the cost of plant establishment. In a wild setting the first purpose is to ensure survival. But even in the wild setting, a component of plant establishment is to outgrow competing vegetation for nutrients, water and sunlight. Hence, rapid growth is generally considered advantageous whether in a horticultural or a wild setting.
The plant establishment period can be defined as the time when a young plant is establishing a root system that is sufficient to support the shoot and is establishing a canopy that is able to gather sufficient sunlight for photosynthesis and is growing the canopy or at least one bud to a height above the reach of local mammalian herbivores.
Poorly designed grow tubes have been known to produce shade-adapted plants, which is problematic if the established plant is to perform in a non-shaded setting (e.g. vineyards). Shade-adapted plants are often characterized by: minimum root development; flimsy stems; overly long internodes; large thin leaves; leaves with insufficient cuticle to protect the leaf in full sun; excessive and dense photoreceptors making the leaves vulnerable to damage in full sun. Inappropriate shade adaptation has been known to cause mortality in grape vines in hot sunny climates. Also, shade-adapted trees are often seen to bend at the top of the grow tube due to poor stem development. Ironically, the shade avoidance response of a tree or vine is often misinterpreted as an increase in growth. The shade avoiding plant allocates stored carbohydrate reserves away from root and stem caliper growth to stem elongation.
A sun-adapted plant typically has smaller leaves with a thicker protective cuticle, a thick stem and good root development. In general, the root and shoot are in balance. The primary controlling mechanism is the red/far-red light balance as detected by the phytochrome pigment system.
Grow tubes substantially enhanced the growth of oak seedlings in the moderate climate found in United Kingdom. Not all climates, however, are as ideal for conventional grow tubes as United Kingdom. In the summer, United Kingdom enjoys plentiful rainfall, cloudy skies that reduce and diffuse sunlight, and temperature maximums below the limits that stress plants. In the winter, it rarely freezes and sunlight is low and diffuse, reducing the likelihood of sun warmed plant tissues emerging from dormancy and then freezing.
In other climates, winter trunk guards are utilized, employing the principles of radiative cooling to prevent uneven solar warming on one side of the trunk and the attendant sun scald and bark cracking of thin barked species such as maple trees. Some winter trunk protection systems comprise radially corrugated tubes akin to field drain tile that are coextruded white on the outside and black inside. The outer white surface reflects most solar radiation. Energy in the trunk is emitted in the form of infrared radiation, absorbed by the interior surface of the trunk protector, and removed from the trunk guard via convection and radiation to the ambient surroundings. By the these mechanisms the trunk temperature stays close to ambient and avoids large temperature differences from the sunny to shady side of the trunk.
Nevertheless, the problems associated with tree shelters and grow tubes often arise from a poor understanding of the stresses on a plant in a confined space and how these stresses are exacerbated by solar radiation. Problems caused by inappropriate radiation include: poor root development; summer dieback and whole plant death; winter dieback and whole plant death; trees with spindly misshapen stems/trunks; a living plant that has nevertheless failed to establish in wild settings; vines that after an initial growth spurt pause for too long.
Currently, some grow tubes include ventilation apertures or perforations that pass through the walls of the grow tube to control temperature and humidity within the grow tube and promote dormancy in broadleaved trees and survival of conifers. A concern is that contact herbicides used adjacent the grow tube will migrate into the grow tube via the ventilation apertures and damage the occupying plant. Accordingly, existing ventilation constructions often force a compromise between effective ventilation and effective weed control with contact herbicides. For example, common practice for tall treeshelters used on broadleaves is to limit the location of ventilation holes to above the spray zone, which may compromise ventilation effectiveness. Shorter tubes used for conifers are completely ventilated, which can preclude the use of contact herbicides altogether.
While grow tubes have proven successful in preventing browsing of certain mammals such as deer and rabbits, they have been known to actually attract other forms of pestilence. For example, wasps have been known to harbor on the interior of grow tubes, creating hazards for attending personnel. Also, rodents have been known to chew through the grow tube near the base and to nest within the grow tube, which can cause stresses on the resident plant including complete stem girdling.
Animals and insects are not the only creatures that pose hazards to grow tubes. Grow tubes are often the target of vandalism.
Grow tubes that address variously the issues of sun and shade adaptation, herbicide encroachment, pestilence attraction and vandalism would be a welcome addition to the horticultural and agricultural industries.