Houseplants: You can start fertilizing houseplants again at this time of year, now that daylight is longer. Generally, you want to hold off fertilizing from December through February due to the short day length and low light levels. The extra nitrogen only encourages weak, leggy growth at that time, and the plants can’t support the extra foliage very well as they are not growing very fast then. However, by late March, days are long enough to start fertilizing again. Fertilizer is important since most of today’s potting mixes have no actual soil in them, and very little in the way of nutrients unless slow-release fertilizer granules were added (check for green or yellow round pellets in the media). If not present, or if the plant has been in the media for a number of months and the fertilizer granules are likely depleted, you can use synthetic water-soluble products such as Scott’s or Miracle Gro (apply as directed), or liquid organic fertilizers such as compost tea or fish emulsion. Bear in mind that the organic products are typically less concentrated and need to be applied more often. Fish emulsion also may be somewhat smelly, so be careful about using this as a fertilizer in high-traffic areas of your house, or if you have cats, who may find it attractive and start digging in your pots.
You may use synthetic slow-release fertilizers for houseplants as well. These are generally sold as granules or “prills” that can be sprinkled on top of the potting media and watered in. These can be very convenient, as they usually are potent between three to eight months.
Some plants such as cacti and succulents need very little nitrogen, and should not be over-fertilized. A feeding once every other month in April, June and August or a light application of slow-release fertilizer once a year is plenty.
Plants such as hibiscus or gardenias have much higher fertilizer needs. Hibiscus should be pruned back by ª to ½ in spring and fertilized every two weeks May-August if using a water-soluble product such as Miracle Gro. If using slow release granules, it would still not hurt to fertilize with a dilute solution (half-strength) of a water-soluble product once a month May-August. A bloom-builder fertilizer is recommended for this use. Start cutting back fertilizer in September and stop in October, re-starting in April. Gardenias prefer an acid fertilizer (such as Miracid) about once a month May-August. Cut back to half-strength in September and stop over winter, resuming in April. If your water supply is alkaline, when the plant is indoors, either use distilled water, de-humidifier water or water with one tablespoon of white vinegar per gallon for watering hibiscus.
— Lisa Johnson, Dane County UW-Extension horticulture educator