Pre-drainage and piercing drain holes
Pre-drainage at the start of the growing cycle is becoming more widespread, which is a positive sign according to Goes. Pre-draining involves piercing a hole two thirds of the way up the slab, so the water content (WC) in the upper part of the slab can drop slightly. The final drain hole is made after rooting in, and no later than when the sides of the first slabs feel dry, draining sooner is always possible.
Lowering the water content while the roots are growing though the entire volume of the slab until the third truss starts to bloom with tomatoes, the first fruit set with sweet peppers, the first heart fruit appears with aubergines or flowering or growth of the first cucumber has now become part of standard growing practice. During this period, little or no water is drained. It is advisable to withhold watering for one or two days during periods of overcast, vegetative conditions.
This strategy results in less to practically no drain effect during the first weeks of growing. Gradually lowering the WC of the slab stimulates the roots to colonise the entire substrate volume with a finely branched root system throughout the slab. The more drip irrigation is applied at this stage, the more roots will form at the base of the slab where the water content is high. A concentration of roots at the base of the slab can cause problems when lots of water is given during periods of high temperatures in summer. This can create anaerobic conditions which kill the roots. Well distributed, finely branched root systems are less easily affected
Different strategy on sunny days
The experiences of growers with restricted irrigation in the initial stage of growing are positive. Especially now, after a long period of overcast conditions, it is however important to follow a different strategy on sunny days and drain well so the water in the slab can be refreshed and to lower the slab EC and pH, as high pH values make it difficult for the plants to absorb various nutrients.
On sunny days, a small volume of water is still being applied via drip irrigation compared with the slab volume. Draining 50% of the equivalent of a coffee cup of water is still very little. A noticeable effect on refreshing the water and lowering the slab EC and pH will only be seen when 1-1.5 litres of water per m2 is drained. The primary goal now is not lowering the slab EC, but efficient refreshment of water instead. Reducing the EC is only necessary when ventilation in the greenhouse is increased and the crop's evaporation capacity has to be boosted.
So the advice is to take things slow and steady in vegetative (overcast, rainy) conditions to encourage healthy root development, but to take well thought out and decisive action on sunny days.
If you can, or have to, drain a considerable quantity of water on a sunny day, start irrigation on time in large - or larger- drip sessions so the slabs reach the resaturation point quickly. Irrigating using more water also encourages moisture intake and evaporation, which means the drainage moment will often be reached later than expected. Starting irrigation on time with large sessions will start the drain moment sooner so that growers quickly gain insight into the actual drain volume. This strategy means you can still stop irrigating early in the afternoon if the drain moment hasn’t yet been reached and you have missed the window of opportunity to significantly refresh the ‘old’ water in the slab.
Extra caution is also required when the vents open at higher outside temperatures. The evaporation rate of the crop may quickly increase unexpectedly, due to higher moisture exchange with the outside air. Adjusting the radiation sum (smaller initial session) is then necessary to increase the frequency of drip irrigation.
(C) Greenhouse Horticulture Waterproof - Author: Jan van Staalduinen