MAR 6, 2013; Brian Callaghan gives advice on growing vegetables in different conditions
Brian Callaghan gives advice on growing vegetables in different conditionsIn addition to space, plants need light, warmth, food and water in order to grow. These resources are usually available all year round in the UK, just not always in the desired amounts, and here lies the fundamental challenge to all growers My vegetable area, for example, is south facing, has 450 – 600mm depth of decent topsoil, is slightly sheltered, without being shaded, and at 140 metres above sea level, avoids the worst effects of frosts. The major drawback is that it has been used for vegetable growing for many decades which has allowed many of the serious plant diseases to develop to damaging levels. Along with careful attention to rotations and a constant lookout for problems, I have stopped growing most brassicas and also now grow only blight-resistant potatoes. The situation is always more fifty shades of grey than black and white with all sites having positive and negative characteristics for growing. The trick is to identify these and then work the site to produce the crops you needs First Step – assessment Even areas currently down to vegetable production will benefit from a regular reassessment as needs and cropping change and soil is always in flux. The law of the minimum states that the factor which falls below the minimum required will define overall growth. In the case above the restricting factor is not soil depth, weather or altitude, it is soil health and the growing has to be adjusted to accommodate that. So what other factors are worth considering? Climate and Micro-Climate Climate is a function of many things but the UK tends to be dominated by relatively mild summers and winters with occasional extremes, plus too much water in the winter and too little in the summer. Far more important is the local climate which is determined by a number of factors. Latitude As a very rough rule, and with some very notable exceptions, the farther north you go the more difficult growing becomes with lower light levels, temperatures and shorter seasons restricting the range of crops which can be grown. It is not grim up North, just often more challenging. Altitude In addition to increasing the likelihood of higher wind speeds, lower temperatures and harder winters, altitude has a marked effect upon the activity of insects that prefer the calmer flying conditions found lower down. This a major consideration for growers of any crop dependent upon insect-pollinated plants such as squashes and pumpkins. Aspect The aspect of a site is its position relative to the sun. A south-facing site, which also gently slopes towards the sun, receives plenty of light energy which will eventually be translated into plant growth. Earlier, daily warming of the soil hastens photosynthesis and dries foliage and fruit of overnight dew helping avoid disease problems. Overall heat storage in the soil is also greater over the season thus delaying the effects of frosts and effectively extending the growing season by up to two weeks. Be aware that slopes can cause problems however, in that heavier, cold air tends to find the lowest point, so avoid growing low down or anywhere where barriers might impede the downward flow of the cold air. Rainfall In most years the UK gets too much rain in the winter and too little in the summer for ideal plant growth. Excessive rainfall hinders timely soil cultivation, especially where machinery is involved, and this delays sowing, planting and, ultimately, harvesting. However, the effect that water has within the soil is more critical, as was highlighted during the wet summer of 2012. Soil is made up of particles between which lie spaces. For the most part each particle is covered with a film of water accessible to roots while leaving room for air between these water-covered particles. When soil becomes waterlogged these air gaps also become filled with water and if this occurs for long periods, plant roots die due to lack of oxygen and growth suffers. Secondly, heating waterlogged soil consumes more energy than heating well-drained soils so wet soils stays cooler for longer, roots grow less and again growth suffers.. As it is usually cloudy when it rains there is an accompanying reduction in the amount of solar energy reaching the soil which makes things even worse. Thirdly, many common plant diseases are always present throughout the year but not in sufficiently high numbers to cause serious damage until conditions are suitable. Potato blight, for example, normally does not appear till around September when days shorten and humidity rises. In 2012 it was sufficiently damp for this infection to cause serious damage to crops in many locations as early as June. Although there is little that can be done to prevent excessive rainfall, much can be done to allow soil to drain more freely. This can range from installing land drains across a site – expensive and/or time consuming work, to raising the level of organic matter within the soil via the addition of compost, animal manure and green manures. Raised beds help keep plant roots above waterlogged soil and timely and careful cultivations such as double digging in autumn improve both the workability of the soil and drainage from the upper reaches. Similarly, delaying sowings of crops such as beetroot by a fortnight allows the soil to dry and may mean the difference between success and failure. Temperature Patterns There are two influences here. The first is frost damage. An air frost is measured 1250mm above the ground so the temperature at ground level will be even lower. For most growers direct damage is likely to occur with a `grass frost` – any temperature below zero degrees measured on turf at ground level. The damaging worst frosts are the first and the last. The first tends to wipe out any tender summer crops overnight. The last frost of spring burns the tops off all the newly emergent material. So, either grow all susceptible crops under cover, apply temporary covers of fleece during the early stages of growth or sow indoors and plant out later when the risk of frost has passed. A mix of these methods cab both widen the range of what can be grown and helps extend the cropping season for many crops. The second influence is on growth rate. Most plant growth becomes insignificant when soil temperature drops below 6°C. Reducing wind speed can have a marked effect on the air and soil temperature and consequently plant growth. When erecting windbreaks assume a factor of eight with, for example, a one metre tall structure providing protection for the eight metres of ground on the leeward side, reducing heat loss from plants and the soil thus allowing for earlier sowing than would otherwise be possible. The perfect growing area is a myth, or an absolute, and as someone quite famous once said “Only the Sith believe in absolutes”. It is, and always will be, a case of using skill and knowledge to make the best use of what is available. Otherwise there would be no challenge and therefore no fun.