Growers can prevent issues related to excess fertilizer and misting with proper planning.
Photo: Dr. Ryan Dickson

As growers become busier and busier in the spring, and the margin of error gets smaller and smaller, it becomes even more important for growers to make sure every part of their growing efforts is exact. This may be especially true for propagators who are working with unrooted cuttings.

The dangers of doing too much or too little

According to Dr. Ryan Dickson, an extension specialist at the University of New Hampshire, a common problem greenhouse operations make in the spring is over-misting cuttings. He says this means giving plants too much, too soon before they develop roots.

“When you over-mist early in production, there is potential to leach fertilizer nutrients from the liner,” Dickson says. “There is also the danger of water-logging the substrate, which lowers the amount of oxygen at the base of the cutting and delays rooting.”

When this happens, Dickson says, heavy leaching can deplete nutrients in the root zone.

“When you receive unrooted cuttings, the plant is essentially dying and it's your job to nurse it back to health and produce a high-quality liner with maximum potential for the next grower,” he says. “During the early stages of propagation, it is a fine balance between too much and too little mist. You continue to make adjustments as the plants develop, so it takes a grower that is attentive and conscientious.”

Dickson says the downsides to applying too little mist is that there a higher risk that the cutting will dry out, as even a little wilting can delay rooting. Problems with under-misting can be less forgiving; it is common for growers to over-apply mist as insurance.

If growers overcompenstae for high pH levels, they could put extra stress on still-developing roots.
Photo: Dr. Ryan Dickson

Monitor pH levels

According to Dickson, pH in growing media can also rise in propagation if plants are over-misted and high leaching occurs.

“Nutrients in the media help stabilize pH. If those nutrients are leached out from over-misting or watering, then the pH can rise to above optimum levels,” he says. “This creates two issues. The first is that there are low nutrients for plant uptake when rooting occurs. The second is that as pH increases, the solubility of certain micronutrients, like iron and manganese, decreases and become unavailable for uptake. If you find yourself in a situation where you have nutrient-deficient and yellow-looking plants, high pH and low nutrients in the media, a simple first step is to just add fertilizer and raise nutrient levels in the media. This will supply nutrients to green up the foliage and also help drop the pH, increasing iron and manganese availability.”

Preventative measures

To fine-tune a misting program, Dickson recommends spending time in the greenhouse watching plants and misting. Ideally, he says, growers should mist the foliage once it dries, but before the plant wilts. If a grower is misting when the foliage is still wet, or if plants are wilting, that is a sign of a problem.

“You wean the plants off,” he says. “And once the plant develops roots, you shouldn’t be misting at all.”

Dickson recommends monitoring pH and nutrient levels at planting to determine if nutrients have been leached out and to determine if fertilizing is necessasry. Dickson also recommends checking pH and EC levels regularly. He also says to check any new crops, or crops that may be more susceptible to nutritional issues, with regularity. Two plants that Dickson says may be more at risk are petunias and calibrachoas.

“Those are both vigorous crops that are sensitive to low nutrients and also sensitive to high pH,” he says. “Also check crops that take longer to root, such as osteo and scaveola. They often require more time under mist, and so there is greater potential to leach nutrients out of the media before the cuttings develop roots.”