Houseplants that are grown for their flowers, need a different type of Fertilizer than houseplants that are grown for their foliage effect.
Houseplant fertilizers for feeding plants indoors are many and varied. They all contain the main ingredients of N-P-K. N=Nitrogen P=Phosphorus and K=Potash. Some plant feeds also contain other elements of food - Trace elements. We will not deal with those here.
All plant feeds should have the symbols NPK with their percentage rate on the packet or container. The higher the % rate the more of that particular feed element there will be in the houseplant fertilizer.
Nitrogen, is generally needed for good healthy foliage, Phosphorus is needed by houseplants for growth of healthy roots and flowers, and Potash is generally for the overall wellbeing of the plant - healthy, sturdy stems and plant framework.
Houseplants need feeding quite soon after feeding, for the feed in most composts will be exhausted after just a few weeks. (This is also true of general composts!). With plants grown indoors, there is a regular watering regime, which aids the loss of nutrients from the root zone, and also bear in mind that the compost is not replenished with re-cycled food - such as dying organic matter - as normal soils are in the outdoor environment. We have to provide the Feed for the indoor plants.
High Nitrogen feeds are often recommended for foliage houseplants. In an indoor artificial growing environment, this can lead to overly lush growth, which in turn leaves the plant susceptible to a wide range of fungal diseases - botrytis being among them.
Bearing in mind that most houseplants will have very little of any of the mentioned elements - NPK - of substance in their pot soil after a month or so, it is normally quite alright to go for just a general liquid feed. No need for high Nitrogen or Phosphorus in most cases.
A general houseplant liquid fertilizer will do the trick for most plants. Another favoured alternative, is to use a weak solution of a Tomato feed. This will typically have a ratio of 7-7-7 NPK or similar, together with which most will also have traces of other elements such as Magnesium. The great thing about Tomato feed for houseplants, is the fact that tomato feeds have been formulated for use in composts (Growbags etc. ) where the growing requirements are very similar to houseplants in compost!
An alternative to liquid houseplant feed, is either plant sticks - which are simply pushed into the pot around the edges - or Osmocote, which is a granular fertilizer which can either be incorporated into new potting soil, or can be bought as 'single plant' dose capsules to be pushed down into the soil. The great thing about Osmocote, is that it will feed the plant for the whole growing season. best incorporated in early spring.