How Changes in Season Influence Greenhouse Plants

With summer plants growing well and soon autumn sets in, growers must be ever-watchful to react smartly to all the changes taking place in the plant and climate over this transition period. During the winter - spring – summer transition, changes are often so subtle that they can catch the grower unawares. It can be generally stated that successful greenhouse growers are those who can “pro-actively” (vs. reactively) avoid potentially harmful conditions to counter potential plant stress and harvest reduction. The main players in the seasonal changes affecting the plants will be lengthened day length plus increasing day and night temperatures. These increases in climatic energy levels equate to increased water use by plants, mainly for evaporative cooling. In the case of indeterminate growers like tomatoes (that were transplanted for the summer), a further complicating factor is the exponential plant development associated with the first rapid plant growth until roughly the onset of the harvest period. During winter, fertigation EC’s are maintained at a relatively high level in an attempt to “force” as much nutrition into the plants during these periods of lower water usage. Inversely, the EC should be gradually reduced as the warmer season progresses due to the higher water usage. If the Ec is not reduced in time, plants may reach a “salt saturation point” and “shut down” transpiration temporarily. This is to protect the cells from damage due to excessive salt build-up in the plant. Typical wilting during the midday period can be ascribed to this “shutdown”. Blossom end rot often results from this periodic wilting. At the same time as reducing EC, the water volume will have to be increased to match the transpiration of the plants. The plants’ water volume requirement will have to be upgraded daily due to the warmer weather as well as the expected rapid growth of young preharvest plants. This increasing need for water is echoed by the growing need for minerals as the plants need more and more nutrients to fuel the rapid growth. The relative nutrient balances must be maintained over the growth period, with careful attention for the need in Potassium during and after flowering – especially in fruiting vegetables like tomatoes.

With high radiation conditions, excessive root EC or salt build-up is one of the main factors resulting in production losses. E.g. Tomato plants with thick stems, compacted internodes, deformed growth points and poor or no fruit set are examples of a too high salt load on the plant. In the broad, growers tend to overfertilize in an attempt to boost growth. Sometimes high root zone salinity can also be ascribed to a watering regime not suited to climate, or simply, inadequate water volume to facilitate sufficient flushing of the root zone (runoff % too low). EC levels should be as low as will maintain good growth and allow plants to continue transpiration throughout the day. Fertilizer mixtures containing significant levels of Chlorides (often in an attempt to lower the cost of fertigation) increase the relative EC of the mixture.

Programs with no or very low Chloride levels, especially for summer, will help in reducing the ion load on the root system – allowing easier uptake of beneficial minerals and further aiding the prevention of salt build-up in the roots. Successful warm weather fertigation can only be done following a program which allows the plant to optimally utilize water for evaporative cooling, transport of sufficient and balanced nutrients plus enough oxygen for healthy root function and growth.

Source: Advice from P de Vries