Within each of the production areas, there are local gradations of climate which result from differences in altitude, topography, proximity to the coast and the presence of natural shelter belts such as woodland. The microclimate in these areas may be further modified in one of the several ways.
The use of shelter and windbreaks
Windbreaks or other forms of shelter are especially useful for vegetable seed production where individual blocks of a seed crop are not always as large as those for agricultural crops.
Solid barriers such as walls divert the windflow, but cause turbulence which can damage the plants. However, windbreaks that offer approximately 50%
obstruction provide a relatively extensive shelter with the minimum of wind gusts. A permeable windbreak can decrease the windspeed in the horizontal direction downwind for a distance equivalent to up to 30 times its height, although most shelter is within a horizontal distance on the leeward side of approximately ten times the height of the shelter.
Shelter belts are also beneficial to the individual plants in the crop because water loss by transpiration and evaporation from the soil is reduced. There is less leaf damage by bruising, and the protective waxy layer on the leaf sur- faces of species such as peas and onions remains more intact. In coastal areas, windbreaks reduce the incidence of scorch from wind-borne salt origi- nating from sea spray. There is usually a slight increase in soil temperature which is especially useful in temperate areas. Soil erosion from ‘blowing’ is also reduced.
In addition to the above effects, the improved microclimate will enhance flower development and increase insect activity and pollen transfer in ento- mophilous crops; the decrease in physical damage to flowers will also result in more fertilization.
Some specialist seed producers believe that the incidence of undesirable cross-pollination between different blocks of cross-compatible crops is reduced when separated by belts of maize or other suitable crops. In practice, ten rows or so of these temporary shelter crops will increase the activity of bee colonies within the separate blocks and so reduce the amount of outcrossing to other blocks. This is especially useful in crops such as aubergine and peppers, where up to 10% outcrossing with other blocks may otherwise be expected. In these instances shelter belts are not a substitute for appropriate isolation distances (see below) but rather enhance them.
Dark (1971), investigating the cross-pollination of sugarbeet, suggested that hedges may affect the incidence of wind-borne pollen in a plot. He proposed that work should be done to see if the sharp drop in pollen concentration on the windward side of a hedge and the gradual build-up again downwind from the hedge were effects of wind deflection or deposition of pollen into the hedge.
There are many different types of windbreaks, but in principle they are either permanent or temporary.
Permanent windbreaks
These are provided by planting lines or belts of single or mixed species of trees that are tolerant of local conditions and have a growth rate suited to quick establishment. A wide range of species is used: evergreens such as conifers andEucalyptus spp. and deciduous species such as Populus spp., Salix spp.
andTamarix spp. Tolerance of the species to salt should be taken into account
where high soil salinity is known to be a potential problem. In many areas appropriate species are planted either along water courses and irrigation channels, or large land areas are subdivided into appropriate size plots by lines of trees.
Temporary shelter
This can be of two types: living materials or manufactured materials. Usually with living plant materials, another crop is grown for its own value and has the dual role of providing shelter for an adjacent crop. Examples depend on the farming system, but Zea mays L. (either maize or sweetcorn, as in Fig. 3.1), Helianthus annuus L. (sunflower) and the cultivated Sorghum spp. are widely used. Occasionally in small-scale operations climbing plants such as Phaseolus spp. on a framework provide shelter.
Other forms of temporary shelter can be provided by erecting screens of manufactured materials such as plastic mesh or hessian. Plastic materials should contain ultraviolet (UV) light inhibitors to prolong their useful life by minimizing UV degradation. These types of structure are relatively low in height but have the advantage of immediate effect and they do not compete with crops for water or nutrients.
Possible disadvantages of living windbreaks
The roots of the species used may enter water courses and drains with the result of water loss, blockages or structural damage to pipes and conduits.
Dense windbreaks can reduce the available photosynthetic light and will com- pete for water and nutrients.
Fig. 3.1. Watermelon seed crop with enhanced environment provided by a maize crop.
Living windbreaks can be hosts to a wide range of pests and pathogens; for example, some Populus spp. are alternative hosts to the lettuce root aphid (Pemphigus bursarius L.), also Salix spp. to the willow-carrot aphid (Cavariella aegopodii Scop.), which is the vector of carrot motley dwarf virus.
Although the relatively dense windbreaks offer shelter to a range of bird species, there is no experimental evidence to suggest that they lead to an increase in crop loss from bird damage, though this can be possible in some locations.
Cover crops
In some areas of the world vegetable seeds are produced as cover crops in plantations. This is relatively common in the tropics and is partly a reflection of the level of seed industry development and partly because of existing agricul- tural crop systems, including a strong tradition to provide shade for some crops.
Young plantation crops such as palms, citrus and banana offer some shelter and in addition there is sufficient arable space for vegetables in the early life of the plantation.