The type of fertiliser you should buy depends totally on the plant. Bring your glasses because you are going to need to read the fine print on the labels of the different fertilisers. You will usually see a list of three different numbers like 10-20-10. These are the percentages in the correct order of nitrogen, phosphorus, and potassium (or potash) comprising the major nutritional ingredients in the fertiliser specified for house plants.
Nitrogen – provides plants what they need to grow and form lush, vividly green foliage.
Phosphorus – needed for strong healthy roots and also promotes flowering and big blooms.
Potassium – needed for growing strong stems and for fighting off various diseases.
There are fertilisers that are formulated specifically for flowering plants and they will have less nitrogen and a bit more phosphorus and potassium in them. There are fertilisers formulated for specific species of plants, like for orchids.
Most indoor house plants require a fertiliser that is balanced like a 10-10-10 formulation.
Plants certainly need the proper amounts of nitrogen, phosphorus, and potassium but that’s not all. They also need tiny amounts of other kinds of nutrients, which are called trace minerals, or sometimes referred to as micronutrients.
When looking at the labels of top quality potting mixes you will find some of these trace minerals. Look for magnesium, calcium, and sulfur and in smaller amounts you may find zinc, iron and copper. If you have plants that you’re not re-potting very often these will need to be fertilized in order to replenish their micronutrients.
The most common fertilisers are water-soluble, which makes they easy to use. They come in several different forms like crystals, powder or liquid. When you continue reading the label you will probably see a recommendation by the manufacturer (in larger print) suggesting you give your plant the maximum amount it can tolerate. Don’t take this at face value because the manufacturer is trying to sell as much of his product as he can. This may very well be too much fertiliser for your plant. As a rule, we at FloristNZ would suggest you use just half the amount recommended by the manufacturer.
The easiest method we’ve found to fertilize your house plants is with slow-release granules. All you need to do is sprinkle them around on the surface of the soil and when you water the plant the granules dissolve.
You can also use fertiliser spikes and they work similarly. All you do is insert the spikes into the soil. You must be careful not to hurt the plant’s roots so although you need to push the spikes into the soil quite deeply, place them near the rim of the pot, away from any roots.
The job of a plant’s rots is to absorb nutrients, but leaves are also able to absorb nutrients. If a plant has suffered a deprivation of nutrients then the most efficient and fastest way to revive this plant is with foliar feeding. When you spray most fertilisers directly onto leaves they will usually become scorched, but foliar feeds are formulated to be sprayed onto the leaves and will not scorch them.
Here are some hints:
New growth is weak
Leaves are pale or faded in color
Leaves are dropping off
Stems seem weak, bend easily
Flowers are small or no blooms at all
You may need to restrain yourself when generously fertilizing your houseplants. More fertiliser certainly does not mean it will be better for your plants. The facts are that many more plants are hurt by too much fertiliser than from a lack of fertiliser. In this case, less is more. Too much fertiliser can burn leaves and roots. If you have any doubt about how much to use, try less.
How can you tell if you’ve applied too much fertiliser?
Look for these symptoms:
Brown areas or spots on leaves or scorched edges
Leaves are misshaped
Leaves are wilting
A white crust is forming on top of the soil or potting mix
Never fertilize a brand new plant or one that has just been re-potted. A plant undergoes stress when it is moved from the garden center or nursery to you home. It needs a good month, maybe two to acclimate to its new surroundings and environment. Your plant will have already been planted in a good quality potting mix, which contains the proper nutrients to nourish it for a few weeks while it adapts.
Again, Florist NZ recommends that you do not use the quantity of fertiliser that the manufacturer suggests you use on the label. They always suggest that people use the maximum amount tolerable for a healthy plan. We normally use just half that amount.
Fertiliser is not a medication for curing a sick plant. Never give fertiliser to a plant suffering from a disease, or that has root damage or even insects.
Next article in the series: Guideline for properly grooming your house plants