This article series is from the Resource Management in Commercial Greenhouse Production Multistate Research Project.
Home gardening of edibles in indoor environments is an important urban trend and a growing market for companies supplying plants and growing systems. This article discusses research at the University of Florida (UF) focused on irrigation, nutrition, lighting, plant material and consumer preference that support “micro-scale” urban agriculture.
Two main growing system options for homegrown edibles are hydroponics or container growing in a root substrate. In hydroponics, the reservoir size is a key consideration. Larger reservoirs require more nutrient solution for replacements, an increased physical footprint in homes, are more difficult to move due to increased weight and have higher costs. However, if the reservoir is too small, more frequent nutrient solution replacements may be needed. For example, in greenhouse trials with hydroponics conducted at UF, four basil plants grown in a 2-gallon reservoir for eight weeks required water replenishment every other day, which is clearly a burden for most home growers. In contrast, a single tomato plant in a 6-gallon reservoir only required weekly top-ups (Fig. 1).
With container production, water and nutrient supply are determined by the growing substrate, container size and water reservoir. Even though a substrate that dries out quickly during the production phase may be easier to grow into a saleable plant, you are not doing the customer any benefit if their plants dry and wilt quickly. Water-holding additives such as gels are options, but the most water you can possibly fit into a pot is limited by the container volume and the substrate water-holding capacity (typically about 65 to 80 percent of the container volume). This water reserve can be greatly increased by providing a reservoir.
Water quality, and the rate at which water and nutrients are taken up by plants, is likely to vary between home growing locations, and most home growers are unlikely to measure pH or electrical conductivity. Therefore, it is important to use a simple and resilient approach to fertilization. Solution pH is likely to drift up or down in hydroponic nutrient solutions, and an iron chelate such as iron-DTPA or iron-EDDHA is needed that remains soluble at high solution pH. Micronutrients should be provided at a higher concentration than in container production because of less efficient root systems and the lack of substrate components such as peat that also provide micronutrients at low pH. Any essential plant nutrient can become limiting, but nitrogen and phosphorus often drive growth rate, whereas magnesium, iron and manganese greatly affect leaf greenness, and calcium deficiency leads to tip burn in leafy greens and blossom end rot in fruit.
An approach to simplify hydroponic fertilization is to provide nutrients in the form of controlled-release fertilizer (CRF) in the transplant you provide to the customer, with the reservoir primarily providing water. A growing substrate provides both nutrient and pH buffering, and is less prone to drift than a nutrient solution.
With container production, performance for the consumer is likely to be improved if the grower provides residual fertilizer that releases over time in the home environment (Fig. 2). Growers can select from several strategies to provide residual fertilizer depending on cost and feasibility for your production system:
- Apply a high concentration of liquid fertilizer immediately before sale. This is cheap, easy to apply and improves performance for a few weeks.
- Top-dress with CRF immediately before sale. This requires extra labor, but CRF dosage equipment can reduce application time. Use a product that will not fall out of the container during shipping.
- Incorporate enough residual CRF to last beyond the production phase. Run trials to ensure that nutrient concentrations are not too high early in production.
- Use a double-coated CRF that begins to release during the consumer phase.
In an indoor environment, photosynthetically active radiation (PAR) is a common limiting factor to plant growth. In commercial production, the goal is to maximize economic productivity, with recommended light levels for lettuce and tomatoes exceeding 15 moles·m-2·day-1 of accumulated PAR. However, it may be unrealistic to achieve this light level in a home environment. In addition, the goal of many indoor home growers is to have a continuous production with an acceptable plant performance and enough yield to enjoy at mealtime, rather than a costly beacon in the living room blasting out purple light through the night.
We ran trials with red-leaf lettuce to determine the minimum light required to grow leafy greens indoors, and recommend at least 6 moles·m-2·day-1 to support active growth and acceptable appearance of leafy greens (Fig. 3). Fruiting crops require more light than leafy greens, but more research is needed to evaluate this minimum light threshold.
Light quality is another important factor to consider when growing plants indoors — blue light is particularly important for anthocyanin pigmentation in leaves. “White” (i.e., phosphor-coated blue) LEDs or fluorescent lamps provide adequate blue light for leafy greens, but lack the far-red light required by long-day flowering plants. As more research is undertaken, light recipes for different plant types will be improved.
Breeding companies are releasing an exciting array of compact and attractive edible plant varieties that are suitable for indoor home production. Two attributes that improve consumer satisfaction are: (a) attractive foliage under low light, which provide interest before flowers and fruit development; and (b) resistance to calcium deficiencies (e.g., tip burn for leafy greens, blossom end rot for tomatoes and peppers) and other disorders.
Plant preparation before sale is also important. For example, rather than expecting a home grower to successfully grow a tomato or pepper crop indoors from start to finish, providing a nearly harvestable plant that is already bearing green fruit may ensure a successful harvest.
Home growers have different goals when they produce an edible plant product. The two types of gardens typically grown, depending on the home grower’s desired outcome, are:
Hobby gardens — Growers are motivated by the pleasure of growing their own food from start to finish, possibly even building their own hydroponic systems.
Cooking-type gardens — Consumers want a constant supply of high-quality fresh produce from either a plant that has already been grown and is ready for harvest or a plant that has been pre-finished and will last one to two weeks and will then be disposed of.
The worst outcome is for consumers to come away with guilt and shame from having a black thumb. If a plant dies, many will consider it their own fault and turn to other alternatives to acquire produce or spend their disposable income. Considering irrigation, nutrients, light, plant material and consumer goals will help ensure a successful gardening experience.