CTAHR NEWS
13 April 2020

Potting Mix, Fertilizer, and Irrigation

The right combo will boost your container garden

Potting Mix, Fertilizer, and Irrigation

Vegetables grow year-round in Hawai‘i, but sunshine and good weather aren’t enough to guarantee success. What happens under the ground, where the roots are developing, is critical to the success of your garden. In this article, we’ll cover how to “feed your food” with the right growing mediums, nutrients, and hydration.

Potting Mix

Commercial potting mixes generally do not contain soil, but rather different materials that promote drainage, aeration, moisture, plant support, and nutrient absorption. They’re ideal for containerized gardens. You can readily find potting mixes at garden nurseries, supermarkets, home improvement stores, or online.

You can also supplement the store-bought mixes with extras from home. For example, topsoil from your garden can be added in a 1:1 ratio—but be careful if soil-borne plant diseases are a problem in your area. You could also add compost, up to 50% of the total mixture. Coir (shredded coconut husk fiber) is another alternative material that can be used in place of, or in combination with, potting mixes.

Fertilizer

One of the most important aspects of successful gardening in Hawai‘i is proper fertilization. A common misconception is that store-bought potting mixes and local garden soils have adequate nutrition to carry all crops to harvest. Unfortunately, mixes often do not come with fertilizer included, and even if they do, they probably don’t have sufficient nutrients to carry the vegetable through its life cycle.

A good general-use fertilizer for containerized plants is Osmocote, which can be incorporated at planting, at a rate of 1 teaspoon to 1 tablespoon per gallon of media. This fertilizer releases nutrients slowly over a long period of time.

General-use granular and dry fertilizers are good options for Hawaiʻi, too. They tend to be fast-acting and are best applied by incorporating directly into the soil. You can also re-apply the fertilizer to the top of the soil line every 2–3 weeks. Foliar fertilizers are water-soluble and can be applied to the soil or sprayed onto the plant’s foliage.

Many organic fertilizers are available to local gardeners. They provide nutrients and contribute to soil health. Organic fertilizers are typically plant or animal based and release nutrients slowly over time as they decompose. The more you garden, the more you’ll learn how to adjust the amount of nutrients for the crops being grown, in order to achieve optimal productivity.

Watering

Daily watering is sufficient for container vegetable gardens. Irrigating in the morning will provide water for plants during the day when they need it, while minimizing excess moisture at night that can contribute to disease. During hot weather and longer days, irrigation may need to be applied more than once per day in order to keep up with plant water demand or minimize heat stress. But avoid overwatering to minimize root-rot diseases. A simple hose-end timer connected to flexible plastic tubing with drip or spray emitters is easy to install from parts found at local garden shops, and can provide water when you are away at work.

At least once a week, give the plants an extra-thorough watering to flush out any excess salts that may have built up because of the fertilizer. Keep in mind that drainage will carry away nutrients that can pollute streams and the ocean, so try to direct drainage to landscape plants and soil, rather than into the storm drain.

For specific questions about pest management, visit the UH Master Gardeners website.

Jari Sugano and Kalani Matsumura, Cooperative Extension Service and Master Gardener Program, UH College of Tropical Agriculture and Human Resources

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