A rainy day in January seems like the perfect time to talk about water. We are often asked about the water we use and how often we water in the school greenhouses, so here are a few of our basic principles.
1.Rain water not tap water. We only ever use rain water at Writhlington. We collect more than enough from the roof of the greenhouse and store it in our 16000litre rainwater tank at the back of the greenhouse.
2. We pump our rain water into a 1000 litre feeding tank in the greenhouse where we add plant food and let the water warm up to greenhouse temperature.
3. We feed with a high nitrogen feed but at very low concentrations. Generally we feed with every other tank of rainwater. This allows for washing out any salt build up. We add half a jar of dry feed to 1000 litre which gives us a conductivity reeding of about 300-400 micro-seimens/cm. This is around 1/4 strength recommended on most commercial plant foods. We water heavily and so week feed often makes sense as well as avoiding a salt build up where water evaporates.
4. We water daily for most of the year but twice daily if the weather is particularly hot and dry (Usually from late May until the middle of July), Watering involves training a hose around the greenhouse from the feeder tank and ‘making it rain’ we focus more water on plants that are mounted, in baskets or noticeably dry. We avoid plants that we have identified as liking it dryer, are in a resting period or are clearly still wet from previous waterings. Plants in pots generally need much less water than mounted plants or those in baskets. Little pots need watering more than big pots.
5. We alter water availability with plant placing and compost selection. A good general rule is that the higher the plant is the dryer it grows. Firstly watering upwards is harder (especially if you are small) and secondly the higher parts of the greenhouse are hotter and dryer. We use a really open bark compost to give excellent drainage of our heavy watering and add dried sphagnum moss sparingly if we want a particularly damp compost (though too much moss risks the plant becoming dry and staying dry)
6. We never damp down. Damping down a greenhouse to keep it cool and increase humidity is widely recommended. We find that damping down wastes water, makes the floors green and dangerous, keeps humidity too high and encourages rots in plants. We like our plants to have damp roots but live in airy conditions where they can photosynthesise effectively and that are comfortable for people too. Our recording of hunidity in tropical forests has shown that it falls to 50% during the heat of the day in and so we do not worry about low humidity in our greenhouse.