Water and watering at the Orchid Project
At the British Orchid Show I promised visitors that we would summarise the way we grow our plants. I will produce a new tab for orchid culture once we have covered the topics of water and watering, temperatures and light, potting and propagation. Here is the first which is an edit of advice we gave in 2017.
A rainy day in November 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. Even in last year’s very hot summer we didn’t run out of rainwater as it stores about four months worth.
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. You will need a lot of water, if you plan to water the plants at home. Try using rainwater and store it in a special tank (read more at
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 but we don’t. 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 humidity in tropical forests has shown that it falls to 50% during the heat of the day in all the forests we have visited and so we do not worry about low humidity in our greenhouse.
I bought 3 new orchids from you last weekend. I know that they need to live in a warm place but please would you tell me what else they need.
The orchids are Prosthechea Cochleata, Prosthechea Aemula and Cologyne Speciosa Albicans. You say Prosthecea Aemula needs “Heavy Watering” but what do you define as heavy watering?
Yes Agnes – all these species like it warm so indoors with heating is fine. Heavy watering means not to let plants completely dry out as the bulbs respond by shrivelling and the you are likely to get poor growth.
I found your cultural notes very informative and I’m sure they will become very popular. I would be most interested to now your opinion about the use of non urea fertilizers and their importance, or not, to absorption by orchids, as opposed to fertilizers containing ureic acid as the main constituent to the nitrogen content. The argument usually put forward for non-ureic content is that it can be absorbed more quickly by the plants as it does not require bacterial breakdown first as is the case for ureic acid. Open bark mixes do not hold the fertilizer long enough for such breakdown to take place apparently. Judging by illustrations of your orchids they do very with your treatment.
I absolutely agree that the nitrogen element of soluble fertilizer should to be non urea (nitrate nitrogen ideally) both for the effective availability of nitrogen and in preventing root burn from the Ammonium hydroxide in urea breakdown. We use Universol Blue https://icl-sf.com/uk-en/products/ornamental_horticulture/2041-universol-blue/ along with the occasional bit of Calcium nitrate (as we use rain water) . However I have found that the heavy watering and weak fertilizing regime we use reduces the sensitivity to particular feed contents. We used to use seaweed based feed but this resulted in deposits on leaves and flowers due to the insoluble ‘brown element’ and we abandoned this type of feed though it is fine if you water plants indoors and don’t spray leaves and flowers with a hose as we inevitably do.
Wow!That’s a really impressive set up. Do you have any data of how much rain water you collect and use?
Sorry for the slow reply – we use around 1500 litres of rain water a week on average which means we have around ten weeks supply in the storage tank which is full at the moment so we have water stored for June and July if the weather turns dry.