Fertilising house plants (plant feeding) is important to provide nutrients which support growth during their growing season. While some growers don’t fertilize plants enough others make the mistake of feeding too much, so it's important to follow some of the basics to get it just about right!
Water soluble: Water soluble fertilizer is the most popular used for indoor plants and simple to use, although this method can be what many growers forget to use or add too much of. This is available in soluble granules, powder, crystals, although liquid is the most popular and compact to store away.
Most plants will need to be fed with this type diluted in water once every week, 2 weeks or monthly, which depends on the plant species and it's needs. This method works well because you have control over when to feed and when to stop, which is important if the plant becomes unwell or the active growth period has ended (feeding ends).
Slow release granules: This has become a popular method of feeding indoor and outdoor plants because it's so convenient. These are placed spread out on the top of soil and gradually dissolve over a 6 month or longer period. I think this is great for outdoor use but indoors I prefer to have control over the period of time my plants are fed. I like to flush plants regularly to keep soil nutrients balanced out, which cannot be done correctly with slow release granules within the soil.
Fertilizer spikes (sticks): Plant spikes are also sold which are small sticks placed in the soil which contain food. I like these because feeding can easily stop just by removing the sticks, but they're not quite as cheap as liquid or granules. One aspect I don’t like of using sticks (just like any other feed which cannot be diluted) is food supply is given to one area of the soil more than another which is uneven compared to the even distribution that soluble types provide the root system.
Orchids, bonsai’s and others: Bonsai’s, orchids and other plants have fertilizer sold specifically for that plant. They can be useful to purchase in a sense because they have the correct amount of nutrients for that plant type. However, fertilizers can be bought which have the correct amount of nitrogen, phosphorus, potassium and other micro-nutrients for some of these plants without buying the more expensive bottle sold specifically for them - or a higher amount can be diluted than suggested on the label. For plants like the African violet (has to be urea-free) or others just mentioned you may just want to go with a specific fertilizer sold for them - to keep it simple and to have the best formula.
With orchids a balanced 10-10-10 fertilizer diluted is pretty good, but with bonsai’s the percentage of elements a bonsai needs is going to be based on the species, the age of the tree and the time of the year, so a little more knowledge is needed for the species a person owns.
It's useful to know what fertilizers contain, the amount needed for various plants and what these do for a supporting and encouraging growth. When you purchase fertilizer from a store the packaging will state the amount of (macro-nutrients) nitrogen, phosphorus, and potassium in percentages (e.g. 20-20-20 or 5-5-5) and then micro-nutrients in much smaller percentages.
When you see advise for a plant to be fed with a diluted balanced fertilizer it means an equal amount of the three main components (nitrogen - phosphorus - potassium).
Micro-nutrients: Various micro-nutrients are also part of a fertilizer formula which are usually a small amount of boron, copper, manganese, zinc and others. These are only needed to be supplied in very small quantities.
Note: The majority of house plants thrive well with a balanced fertilizer, especially foliage types.
Most plants follow the standard procedure of being fed during the growing season from spring - fall, with fertilizing completely stopping during winter. Some plants can be fed at other times or throughout the year (mainly winter flowering types).
It's sometimes overlooked or not even known by many growers that the majority of plants need a rest period. This is a time for feeding to stop and watering to slow down. If your plant at the beginning of winter ( October - November) shows signs of no further growth it's aware that winter is here and feeding can stop.
New and re potting: When a plant is bought or re-potted with new soil you will not need to use fertilizer for the next month because of the new potting mix already containing plenty of nutrients. It's better if your plant gets less fertilizer than too much.
Poorly plants: Plants become unwell at times for a variety of reasons which can lead growers to believe feeding will help, when this is a time to stop feeding and try and diagnose then rectify the faults. The problem with a plant could actually be over fertilizing so the last thing it needs is more food. The worst that can happen when fertilizer stops is slower or weak growth and smaller flowers or no flowers, which can easily be put right once other problems are resolved.
How often and amount: It's best to follow the guidelines for feeding times of the brand or product purchased, which is usually once a week or two during the growing period. However, do follow a guide (similar to the guides on this site for the specific plant) because a lot of plants do not need feeding every week and only want half the amount diluted that's recommended on the label. Also, plants which are growing in low light conditions do not grow as quick - so less feed is required.
It's better to feed too little than too much, as you will see with the potential problems caused either way. Overfeeding causes more serious problems than not providing enough. It's important to be aware that the list of problems below could also have another cause unrelated to fertilizer, so you must check all other potential causes as well.
Not Enough- Slow and weak growth
Too Much- Slow and weak growth
If over fertilized: If a plant has the above problems from over-fertilizing you can flush the soil with just water so all salt build up can clear out, and give the plant a month or two before feeding again. Another option is to replace the potting soil and do not use fertilizer for about 2 months.
Under fed: If under fertilized simply begin feeding based on the requirements for that plant species.