A major challenge faced by glasshouse vegetable growers in warmer climates, is dealing with the hot conditions, during the day and at night. High temperatures and radiation can create problems. So, the question becomes: what can be done to prevent these conditions from creating excessive stress on the crop?
By Phil Johnson
In the warmer months, the first thing to consider is your goals as a grower: what do you aim to achieve. Through the season, generally the goals for the grower are uniformity of climate, irrigation and therefore the crop itself. Early on the grower should aim to produce steady growth so that the crop has the right balance going into summer. The balance we talk about is between generative and vegetative, strong and weak, parameters which can be assessed through simple measurements of the plant.
Planning and the continuous assessment of the crop throughout the season will help towards achieving these goals. As part of the planning process growers should aim to identify the major stages/events during the cropping cycle, making observations of how the crop may react in relation to high radiation and temperatures, and assess the potential risks to the crop at each stage. From these observations an action plan can be formulated and implemented, if necessary, so as to reduce or prevent the risks, which have been identified. Table 1 illustrates such an action plan for an over-wintered crop planted in late summer with key stages of the production cycle as column headings. Observations made in the previous season have been used by the crop consultant to stimulate the mind of the grower (i.e. what was observed in the crop and what risks these presented to production/quality). From here a list of actions is suggested from which a more detailed action or growing strategy can be devised in order to eliminate or reduce the risk in the following season.
If you are aiming to plant your crop in hot conditions, consider that there are risks of Pythium and Blossom End Rot (BER). Planting the crop directly on delivery from the propagator, rather than standing it next to the plant hole, will greatly reduce these risks. The outside climate in summer is generative enough and we do not require generative actions. Planting directly will allow the crop to root in quickly to the slab. It can therefore make leaf area which will shade the slab (i.e. cool the root zone and prevent the risk of Pythium).
Moreover, the crop will start to develop its own climate (humidity) faster. Shading the crop will also help to take some of the pressure off the crop by keeping the inside climate more controlled at a time when the small crop is unable to create a climate due to its size. For a crop planted in summer there is a lot of assimilates available at the start of production due to the high outside light levels and low fruit load. So, in order to keep the crop balanced it is advisable not to give C02 until the fifth truss is setting. Consider using truss supports as both high day and night temperatures will make them weak and prone to kinking, which may affect fruit size and quality.
Work on speed to build a fruit load by maintaining a high 24-hour temperature (23 degrees C) until the third truss; but with a pre-night drop of 17-19 degrees C if possible. This will help calcium uptake from the beginning of fruit development to help overcome the problem of BER.
From the third truss you can then go down in temperature according to the general rule of thumb: 17 degrees C+ (Joules measured – (100J + (number from the moment of flowering of the third cluster go down with the 24-hour temperature according to clusters*100J))/300.
Example: if you measure 1,200 Joules per day and you have five trusses per plant, the average 24-hour temperature should be:
17+ (1200-(100+(5*100))/300 = 17 + (1200-600)/300
= 17 + 2 = 19 degrees C. (24hr temperature)
Keeping vigour in hot conditions
Keeping vigour or strength in the crop will basically involve keeping it as cool as possible. Shading the crop will help, however the timing of shading, the temperature and radiation levels at which they should operate, will depend on the balance in the crop and the quality of the root system (i.e. a crop with sufficient leaf area and a healthy root system will be able to cool itself better than a crop with small leaves whose root system is affected by Pythium).
As well as shade screens, fogging will help to maintain and control temperature and humidity. However, the fogging system must not make the crop wet; this will burn the leaves. Correct fogging can also be used to improve pollen quality/setting as it can be used to reduce the speed at which humidity levels fall in the morning.
The potential positive and negative effect of having the correct or incorrect irrigation strategy can be underestimated. It is important to have a substrate which can be monitored and therefore controlled at all times during the cropping season in order to steer the crop according to plant stage and climate.
It is important to have a well rooted plant, which is functioning well, so it is important to build a strong root zone for the summer period. Keep a stable root environment by irrigating on light—have EC under control by 10:00 am to aid water uptake during the peak period of light.
Starting and stopping. Remember one golden rule, ‘transpiration before irrigation’. Watering an inactive crop will create root pressure which may split fruit and stem cells, thus leaving plants open to Botrytis attack. If you have a Grodan water content meter, plant activity can be measured in the morning. It can be seen on the graph where the water content begins to fall sharply shortly after sunrise as the plant begins to transpire and take up water naturally.
Having the correct substrate volume is also important for being able to achieve this without losing control of the slab water content and the slab EC. A low substrate volume may make changes in water content and EC larger and faster, making control more difficult.
Grafting is mainly used as a disease control/prevention measure but may be a consideration for hot climates. Grafted crops have more vigour in the summer months due to the stronger root system. Grafting does however make the plant more vegetative in winter time on days with radiation levels below 1500J. Therefore, you need to consider using a different feed recipe (low nitrate, high sulphate and high chloride; a higher dripping EC; a temperature boost around midday; and labour (de-leafing) to keep the crop in balance.
Stopping the crop in hot conditions
Stopping a plant, by removing the growing point prior to the end of the production cycle in hot conditions poses additional risks to fruit quality—namely, fruit cracking (russetting) on the last trusses. This disorder is caused by a lowering (or low) fruit load and excessive available assimilates (sunshine). The fruits grow too quickly. Using shade on the roof or screens and stopping de-leafing and truss pruning will protect against this.
Remember, controlling just one of the areas mentioned here will not necessarily eliminate or prevent an issue from arising. It is important to have balance and control in all areas of the crop to be more confident of avoiding any problems.
About the author
Phil Johnson is the technical adviser New Zealand and Australia for Grodan. Ph: 0064 27 2971779, Website: www.grodan.com Ω
PH&G March 2016 / Issue 165