Garden calendar | For the week of April 14
Houseplants: You can start fertilizing houseplants again at this time of year, since days are longer. Generally, you want to stop fertilizing from October to March due to the short day length and low light levels. The extra nitrogen would only encourage weak, leggy growth at that time, and the plants can’t support the extra foliage very well then due to the short days and low light. An exception to this would be if you use grow lights as your major light source 12 hours a day or more year-round, since the light levels would be the same year-round.
Fertilizer is important because most potting mixes (media) have no actual soil, and few nutrients unless slow-release fertilizer granules were mixed in (check the bag label and look for colored, round pellets in the media). If not present, or if the plants have been growing in the same pot for some time, without fertilizer, nutrients are likely depleted.
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Useful fertilizers include synthetic water-soluble products such as Scott’s, Miracle Gro, Jack’s or others — apply as directed — or you can use liquid organic fertilizers such as compost tea or fish emulsion. Organic products are typically less concentrated and need to be applied more often. You can also use synthetic slow-release fertilizers for houseplants instead of liquid; they are generally potent for three to eight months — it should say on the label. These are mostly sold as granules or “prills” that can be sprinkled on top of the potting media and watered in. The label should give directions as to how much to apply based on pot size and/or plant species.
Some plants such as cacti and succulents need very little nitrogen; do not over-fertilize. A dilute feeding once in May and July or a light application of slow-release fertilizer once a year is plenty for them. Orchids have their own special fertilizer that should not be over-used. Most other tropical houseplants can be fertilized once a month April through September.
Tropical blooming plants like hibiscus have higher fertilizer needs. Prune back hibiscus by ⅓ to ½ in spring, and if outside, fertilize every two weeks May through August if using a water-soluble product such as Miracle Gro or once a week with an organic product. If using slow release granules instead outside, it would still be good to fertilize with a dilute solution (half-strength) of a water-soluble bloom builder fertilizer once a month in addition to the single application of a slow release product May through August. If the hibiscus is inside the house during that time, once a month is fine for synthetic fertilizers and twice a month for organics. Start cutting back fertilizer in September and stop in October, restarting in April.
— Lisa Johnson, Dane County UW-Extension horticulture educator
— Lisa Johnson, Dane County UW-Extension horticulture educator
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