3 tips for holding spring crops

According to Christopher Currey, a professor at Iowa State University, holding crops is not a good fit for a coronavirus-impacted spring.

“Life really does just happen,” he said at a webinar he hosted at Cultivate’20 Virtual. “In greenhouse crop production, we talk about scheduling, scheduling, scheduling. But sometimes, things change. This spring is a perfect example, with COVID-19 and all of the disruptions we’ve experienced in our lives.”

“But unexpected things happen all time,” he continued. “What if you have cold shipping temperatures and can’t get young plants on the truck in early spring. Or what if there’s a shipping issue and you can’t get plants out of the greenhouse at all?”

Here are three tips for holding spring crops. He noted that none are “silver bullets,” but all options to hold plants.

1. Modify air temperature. Currey said that holding air temperature can have the most “immediate impact” on crops.

“What temperature we hold our crops at varies,” he said. “When talk about holding spring crops, it can be a lot of more challenging because we are growing lots  and lots of different species, not just a greenhouse full of poinsettias.”

Currey says the key to is to know the base temperature and the optimum temperature and work from there. If the temperature exceeds that optimum temperature, a stress reaction will stall out plant growth with the development rate slowing more and more as the temperature rises.

“Plants can’t regenerate fast enough under those hot temperatures,” he said.

Plants for this can be classified into the following categories for holding:

2. Think outside. For cooling crops down, a greenhouse is a good choice, Currey said, because of the ability to control the environment via a control system and/or a retractable roof. However, he also recommends going outside to cool crops down.

“This is a tool we can use in the spring and not in the fall and later months, depending on your geographic location,” he said. When outside, he recommends putting cloth over plants and/or running sprinklers at night to prevent freezing. The latter is a trick used by nursery growers and orchard growers.

3. Trim your plants. According to Currey, trimming or pruning plants achieves two goals for holding.

The first is that it reduces plant size and avoids overgrown plants. Secondly, it prepares plants for a later sales date with flushes of new growth.

“That flush of growth will make it more marketable and have it looking nice on the shelf,” he said.

Equipment for this, Currey said, includes everything from machinery designed for trimming, standard shears and even hedge trimmers. But before pulling out the shears, Currey said it’s important to keep a few things in mind before going, as he put, “all Edward Scissorhands.”

“Ask yourself ‘what is the growth rate of the plant?’,” he said. “Generally speaking, pruning and shearing are more suitable to plants with a prostrate growth habit, that spreading, mounding appearance, as opposed to something upright.” As examples, Currey cited a verbena as a plant with a prostrate growth habit and a sunflower as a plant with an upright growth habit.

“Think about what the pruning and shearing is going to do and think about what it’s going to look like when the regrowth comes,” he said.

He also recommends knowing where the flowers are going to be, meaning knowing if the plant has Axillary flowers or Terminal flowers. Typically, a plant with a prostrate growth habit will also have Axillary flowers, while an upright habit will have Terminal flowers.

“If you go in and shear off the growing points on a plant with terminal flowers, that [regrowth] is going to further delayed,” he said.