Many people remember to water their houseplants but don’t consider fertilizing them. It is important to fertilize your houseplants for two reasons: 1) plants need certain mineral nutrients for overall health; and 2) watering leaches away any nutrients in the potting mix. Remember that the plant has been or will live in its container for years and has no way to receive nutrients, like a plant would if it were living outside. Some nutrients come in your potting mix but each time you water the plant, you flush some of the original nutrients away.
Fertilizer vs. Plant Food
As important as fertilizer is to plants, it is not plant food. Commercial houseplant fertilizers are often called “plant food,” but plants make their food through photosynthesis. Fertilizers provide the nutrients necessary for the plant to function overall as well as to photosynthesize efficiently.
Macronutrients in Fertilizers
Fertilizers contain the mineral nutrients a plant needs. The three mineral nutrients a plant needs the most in terms of quantity are nitrogen, potassium, and phosphorus. These are also called the macronutrients. A fertilizer will list the ratios of these macronutrients in nitrogen, phosphorus, and potassium order such as 10-5-9 or 10-10-10. These are ratios, not percentages.
Nitrogen
If the fertilizer has a higher N number than the rest of the numbers, such as 10-5-9, it has more nitrogen than phosphorus or potassium. Nitrogen is elemental for increasing leaf production and healthy foliage growth so a fertilizer with this ratio is desired for tropical foliage houseplants.
Phosphorus
Phosphorus is essential for all plants but it particularly helps to promote bud formation, flowering, and fruiting. A fertilizer with a higher middle number such as 1-3-1 is used for flowering houseplants like African violets or orchids.
Potassium
Potassium supports the function of roots and the ability to transport moisture and nutrients to all plant tissues.
What Are Complete and Balanced Fertilizers?
A “complete” fertilizer has all three, N, P, and K. A “balanced” fertilizer has the same number for each such as 7-7-7 or 10-10-10.
Micronutrients in Fertilizers
Plants also need other minerals although in lesser quantities. These are called the micronutrients. No less important, just not as much is needed by the plant. Of the micronutrients, calcium, magnesium, and sulfur are needed most, followed by iron, manganese, zinc, copper, boron molybdenum.
When To Feed a Houseplant
To determine when to feed a houseplant, it is best to learn about that plant as much as possible. For example, flowering houseplants will require a different formula than foliage plants and succulents and cacti do not need as much fertilizer as the aforementioned.
In general, apply fertilizer during the plant’s active growing time, which usually is spring to fall. The exception is a winter blooming plant. It is best to develop a schedule, so you remember to feed on a particular day during the plant’s active growing time. Keep in mind that some plants are heavy feeders and of course some are larger than others.
It is not necessary to feed a newly purchased houseplant or one that has been re-potted with new potting mix, because the mix should already contain enough nutrients. However, as time passes and you water the plant, the nutrients will leach out. Think about fertilizing a month later and develop a schedule.
Do not fertilize when the soil is dry, either water first or water with very diluted fertilizer. If you forgot to water and the plant is wilting, water first, and let it recover before you fertilize. Don’t fertilize plants that are stressed by a pest infestation; wait until you have been able to solve that issue first.
Depending on the product you use, you can dilute the fertilizer to quarter or half strength so you feed the plants each time you water (so feed continuously) or you can feed all the plants on a particular day. The exact amount and frequency should be in the fertilizer instructions, but you should come up with a plan or schedule so you do not forget.
Types of Fertilizers and How To Use Them
Fertilizer comes in different forms. It is not that one is better than the other but that you will prefer one over the other based on cost and ease of application. However, there are some that are specifically formulated for certain types of plants such as African violets, orchids, or cacti and succulents. A fertilizer like this already has the correct proportions of macro and micro for that specific type of plant plus the correct dosage.
Liquid Fertilizer
There is ready to use liquid and a concentrated liquid that must be diluted. The ready to use liquid is just added to the water you use to water the plants. It probably has very low numbers such as 2-2-2 and is intended for continuous feeding. The concentrated form will have higher numbers and must be diluted first in water. Depending on how much you dilute you can use for continuous feed or feed on a schedule.
Water Soluble Powder Fertilizer
This is a cheaper option, but the powder has to be measured and dissolved in water first before applying. Some are made with messy dyes that can stain the hands. In the summer, the humidity may make the powder clump. However, because you are determining the dosage, it is easy to cut the strength and add to the regular watering schedule or fertilize on a schedule.
Slow-Release Granules
The granules can be sprinkled on the soil surface, mix in with a fork, and then watered in. Application can be tricky if there is little soil visible because of the foliage. Over time, the fertilizer is released to the roots, so this is done every few months. You will have to remember when you applied the granules. Alternatively, and maybe easier, you can mix the granules with the potting mix but again will have to remember when you did this and to which container, especially if you have many plants. Also, there is a chance you are adding fertilizer to fertilizer that already exists in the potting mix, hence overfertilizing.
Spikes, Sticks and Tablets
These are a form of slow-release fertilizer; they will last a few months. Again, this can be tricky if there is little soil visible because of the foliage. Also, you will have to remember when you inserted the stick or tablet. There is a chance of root burn since the stick or tablet may come into direct contact with the roots.
Foliar Feeds
These are liquids that are sprayed on to the plant. The disadvantage of course is the mess, you may not be able to spray in your living room. However, this is a good method for feeding epiphytes such as tillandsias and bromeliads.
Chemical vs. Organic
The chemical versus organic fertilizer question is more important for fertilizing plants outside in the garden than for houseplants indoors. For indoor plants, this is a personal preference. The plant itself does not care. Remember that the potting mix does not have the microbial and insect life that exists outside in the garden soil so using a synthetic fertilizer is not harmful to the houseplant or the potting mix. Also, you may read that organic fertilizer is environmentally friendly and improves the soil structure, but potting mix does not need improvement nor will the fertilizer affect the environment since the plant is inside the house.
Organic fertilizers are made with organic ingredients such as liquid kelp, fish emulsion, and bat guano. Yes, some can be odiferous. Usually, the three macronutrients are present but not all the micronutrients. They tend to be weaker solutions than synthetic fertilizers. Synthetic fertilizers are made from man-made, inorganic compounds usually derived from by-products of the petroleum industry. They do not tend to be as odiferous as organic fertilizers and usually have both macronutrients and some if not all micronutrients.
More Is Not Better
With fertilizer, more is not better. Too much fertilizer provides too much salt to the plant, adversely affecting the plant’s ability to take up moisture and nutrients. Sometimes you can see the salt as a white crust on the soil’s surface or on the container. This does not necessarily mean you overfertilized, though. Plants rarely take up all fertilizers and often the moisture evaporates in the uppermost top inch of soil or on the container’s rim leaving behind the salts—the white crust. To flush the salts out, watering thoroughly, allow the water to drain, then drench it again. There are some plants that are especially sensitive to salts so you may have to drench every month or so. This is why it is best to learn as much about the plant as possible to see if it is sensitive to salts or any other chemical.
Fertilizing houseplants is just as important as watering them. Learn what your plant needs and what works best for you in terms of your budget and time. Then develop a plan to provide your houseplants the nutrients they need to stay healthy.
