Simplifying spring crops with a group mentality

The roller coaster is clicking up the tracks, and we are not far from that free fall and ensuing chaos. In other words, spring is upon us! The diversity in spring crops is what makes it so exciting and interesting … and perhaps a little stressful, too. But there are approaches you can take to increase efficiency and simplify production. This article will review some of the best ways to group crops for simplifying spring production and improving finished plant quality.

Temperature is arguably the first, and most important, way to group crops for production. The diversity in annual and perennial plant responses to temperature vary. We can group plants into three different categories based on their temperature responses: cold-tolerant plants, cold-intermediate and cold-sensitive. Another way these groups have been described are cool, intermediate and warm groups. Regardless, placing annuals and perennials with similar requirements together improves quality and uniformity.

Ideally, the grouping can occur in separate growing areas and allows for air temperature to be controlled independently. By providing air temperatures that are closer to the different optimal temperatures of the different plant groups, both the rate of development and quality can be maximized. If independent growing environments are unavailable, take advantage of the greenhouse microclimates to group plants. Place the cold-tolerant crops by the pads, where incoming air will be coolest, and cold-sensitive crops on the other side of the greenhouse by the fans, where the warmed air is exhausted.

In addition to temperature, light is another factor to consider when grouping plants in the greenhouse. You may be thinking about grouping plants by their response to daily light integral, such as low-, moderate- and high-light crops. This certainly makes sense, but during the spring we are usually not experiencing supra-optimal light intensities in the greenhouse. Instead, we are usually trying to figure out how to increase it for nearly all our crops. Instead, segregating plants by their flowering response to daylength, or photoperiod, can be useful. In looking at the balance of flowering responses for spring plants, the most widespread is a long day photoperiod response, including both facultative and obligate long day responses.

Trying to force long day plants into flower during the spring can be a challenge because the natural daylengths are too short (or more importantly, nights too long). All that is required is low-intensity lighting that can deliver 2 µmol·m–2·s–1 at plant height to either: 1) extend the day before sunrise or after sunset to create a long day (and short night); or 2) interrupt the night from 10 p.m. to 2 a.m. While the majority of spring crops are long day plants, there are still several important short day (or long night) plants. While there are natural short days during the earliest part of the spring season, the daylength can become too long as the season proceeds to induce flowering. The only way to create short days is to use blackout cloth to truncate the daylength. While those growers who finish poinsettias in the fall may already have a blackout curtain installed for the early crops, many growers do not have that infrastructure in their greenhouses. By grouping short day plants together and condensing them, manually pulling blackcloth over a few benches is simple way to induce short day plants.

Day neutral crops, which will flower under short or long days, do not need to be placed under blackcloth or lighting, but they can be placed under either if it helps organization and blocking.

Finally, one of the last considerations for grouping crops is by their root zone requirements. Annual and perennial garden plants have a variety of fertilizer requirements, spanning from low-, moderate- to heavy-feeding crops. Additionally, substrate pH requirements vary from low (5.4-5.8), general (5.8-6.2) and high (6.2-6.6). While substrate pH can be adjusted before planting to accommodate crop requirements and supplemental controlled-release fertilizer can be provided to plants with higher fertilizer requirements, using different water-soluble fertilizer formulations and concentrations helps maintain rootzone conditions. Again, blocking by rootzone requirements makes fertilizing a more efficient process, requiring less changing stock solutions or concentrations over the course of the day. This is especially true for irrigation systems covering larger areas, such as flood tables or floors, drippers or booms.

A greenhouse may not be a Rubik’s cube, but it is certainly a puzzle in the spring. Taking the time to plan and organize how spring crops are laid out and set down can streamline efforts and optimize crop production. Temperature, photoperiod and rootzone requirements are convenient crop characteristics to group plants by and out the pieces of the puzzle in place.