The aim in the following regime is to give plants the most nitrogen during the very active growth period, cycling through a medium nitrogen period before reducing nitrogen levels to the lowest during the winter months. The products mentioned are from the DynaGro range, as these contain all of the micro and macro nutrients required to promote healthy plants but other fertilisers with similar formulations may be used instead. When choosing fertiliser those containing urea should be avoided for the following reasons; first, urea does not readily break down except in garden soil/compost, so this form of nitrogen is not easily available to the plant in most orchid growing media. Secondly, urea does not register on TDS meters, so those who use these instruments will be misled by the readings if the fertiliser contains urea, and risk over feeding and thus scorching their plants.
The actual timings of the change are largely determined by the weather but the following dates are a good guide upon which to base decisions.
In January and February use a low nitrogen formula such as DynaGro Bloom.
March to mid April use a medium nitrogen formula e.g. DynaGro Grow.
From mid April to the end of July use a high nitrogen formula e.g. DynaGro Foliage-Pro.
During August and September switch back to medium nitrogen e.g. DynaGro Grow.
During October to December use low nitrogen e.g. DynaGro Bloom.
For those with a small number of orchids the best compromise is to use a broadly balanced, medium nitrogen fertiliser such as DynaGro Grow.
Bear in mind that some orchids thrive on more 'food' than others and also the level of feed required may also be affected by the amount of light the plants are getting. Do not overfeed plants which are not growing well - if they don't have much in the way of roots they can't take in a lot of feed. Address the root problem by re-potting if necessary and then resume normal feeding when the plant has a good set of roots.
Next: Geoff Hands' page on Conductivity Readings for advice on feed strength for various orchids.