Indoor Plant Basics: Fertilizer, by Jim Hole

Fertilizer rules and recommendations

Plants need fertilizer to supplement their diets. Although they feed on light and the nutrients in the soil, a boost of fertilizer can help promote and support strong, healthy growth.

Fertilizers contain three major nutrients to support stem and leaf production, flowering and healthy roots. These elements are nitrogen (N), phosphorous (P) and potassium (K). When you look at a container of fertilizer, pay special attention to the fertilizer analysis represented by three hyphenated numbers (for example, 20-20-20 or 10-6-16). The first number represents available nitrogen. The second number represents available phosphate, and the third number represents available potash. The higher the number, the greater the percentage by weight of that nutrient. Plants require nitrogen for leafy growth, so as a general rule, plants grown primarily for their foliage are given a fertilizer with a high first number, a lower second number and a third number that is comparable to the first. Plants grown primarily for their blooms are given a fertilizer with a high third number (K or potash) that promotes flower development. 

Fertilizers are most beneficial to a plant during its growing season (February to October). During winter months there is less light, so hold back on fertilizing unless your plant is showing signs of new growth. A plant’s fertilizer consumption follows its growth curve, which in turn follows a light and temperature curve.

General Rules for Fertilizing

  • Granular and liquid fertilizers work similarly. Be sure to read the instructions and to mix and feed accordingly.
  • Hold off fertilizing for
  • at least a few weeks after plants are repotted. It’s not that the plants don’t need food; it’s that they need only so much. Most soils contain unknown amounts of fertilizer and it’s easy to overfeed your transplant.
  • Water until water flows out the bottom of the container. This step will help flush any buildup of soluble salt deposits. As salts become more concentrated, it becomes harder for a plant to take up a proper supply of water.

Containers

Your plant needs a suitable home to live successfully indoors. That’s why container choice is so important—and there’s a wide range of containers from which to choose.

The two most important factors to consider when choosing a container are size—in both depth and diameter—and drainage. Aesthetics are also a consideration.

Size

Make sure your plant has the proper root-to-soil volume. This means choosing a container that will accommodate a plant’s root system and a sufficient amount of soil to sustain it. An oversized pot holds more soil than is needed, and that soil can easily become saturated with water, disrupting the air/water balance and increasing the plant’s chance of dying of root rot. Never increase soil volume by more than one pot size when repotting.

Drainage

Unless you’re growing an indoor water garden, be sure to choose containers that have drainage holes. Water must be able to drain through the soil and out of the pot. Without proper drainage, a plant is likely to die. If you’re thinking about putting rocks at the bottom of your pot to help with drainage, don’t! It’s a point that you’ll hear repeated in this book—basically because it’s worth repeating. Pebbles shorten the column of soil, allowing the soil to become more easily waterlogged.

Beauty

Just because a container has to be functional doesn’t mean it can’t be attractive, too. Garden centres are full of beautiful containers that fit any style and budget. The right container can make just as big an impression as the plant -itself, so take your time and pay attention to those finishing touches—they have a way of making all the difference.

Tips

  • Remember to buy a saucer or tray to go under a container (many containers are sold with a matching saucer).
  • Add caster wheels to the bottom of a large container for easy mobility.
  • Use decorative moss, pebbles and driftwood on the soil surface to create visual interest and to discourage pets from digging.
  • Conceal less attractive pots and saucers in decorative baskets, crocks or plant stands.

Did you know?

The theory that layers of pebbles at the bottom of a pot is good for a plant is older than it is wise. Pebbles actually hinder drainage by reducing the soil depth. Most people who add rocks to a pot without drainage holes assume that the rocks will create a reservoir for the water to accumulate. While this is true, excessive salts will accumulate in the reservoir over time. Further, when you displace soil space with pebbles, you reduce the amount of living space for roots. To give your plants the best home possible, leave the rocks out of the bottom of their pots.