Too many people overlook the importance of fertilizing indoor plants. However, proper feedings are essential to grow healthy, beautiful plants. Unlike an outdoor garden, where nature provides rain and plants can send new roots searching for food, the nutrients available to a houseplant are strictly limited by the amount of soil in the pot and what you provide for supplemental feeding.
Think of fertilizer as the second half of your potting soil. When potting soil is fresh, your plants won't need much, if any, fertilizer. This is especially true of modern, fortified potting soils, which often have fertilizer and other enhancements mixed in. After about two months, though, the plant will have consumed the nutrients in the soil, so you'll have to fertilize if you want continued, healthy growth.
Types of Fertilizers
Fertilizers come in several different varieties: liquids, sticks, tablets, granules, and slow-release forms. Of these, the two best suited for indoor use are liquid and slow-release fertilizers. Sticks and granules seem convenient, but they don't distribute nutrients very well through the soil, and once you've inserted a fertilizer stick into your pot, you have no control over its release. Granular fertilizers are designed for outdoor use.
Use Liquid Fertilizer
Liquid fertilizers are diluted into water and applied with a watering can. Depending on label instructions, you might fertilize every time you water or every other time. The type of plant will also impact the frequency, as some—especially those with dramatic large blooms—may require more frequent feeding. Always research plant requirements to learn about their specific nutritional needs. Liquid fertilizer provides a steady supply of nutrients that you can precisely control. It's easy to suspend feeding when the plant is dormant during the winter months, for example, or to increase feeding when the plant is sending up new growth. The disadvantage, however, is that you need to remember to do it every time.
Try Slow Release Fertilizer
These products have quickly become favorites for many gardeners and professional growers, both for indoor and outdoor plants. Slow-release fertilizers are coated in time-release shells that slowly leach nutrients into the soil. The individual pellets have coatings of different thicknesses that dissolve at different rates, so the actual release of the fertilizer is staggered over time. A single application can last between four and ninth months. The main drawback is the higher cost of slow-release fertilizer, but because it lasts so long, the cost balances out.
Use Granular Fertilizer
Dry pellets of pure fertilizer can be mixed into the potting soil by hand. Although more commonly used in outdoor gardens, they can also be used for indoor containers—although it can be tricky. Granular fertilizer dumps all of its nutrients at once when the pot is watered, making it hard to control how much the plants are receiving at once. This type of fertilizer is quite inexpensive, but not a great choice for feeding houseplants.
Tips for Buying Fertilizer
All general-purpose fertilizers contain the basic macronutrients that plants need to grow, including nitrogen, phosphorus, and potassium. Each macronutrient has a specific function:
- Nitrogen encourages healthy foliage growth.
- Phosphorous encourages bigger, healthier blooms.
- Potassium encourages a strong room system.
Specialty fertilizers, such as African violet fertilizers, contain optimized proportions of these nutrients for particular kinds of plants.
In addition to these macronutrients, better-quality fertilizers also contain micronutrients such as boron, magnesium, and manganese that will encourage healthier growth. Study the fertilizer label to determine what nutrients it contains.
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Fertilizer for indoor plants. University of Maryland Extension.
Pros and Cons of Granular and Liquid Fertilizers. Michigan State University Extension