MSU Extension

Winter weather can turn any trip into an adventure. Over the past few weeks, a combination of freezing rain and snow across the southern U.S. has caused significant transportation delays on highways and airways alike. Given that shipping season for vegetative cuttings began in earnest several weeks ago, the likelihood that some of these shipments were caught up in the turmoil is almost guaranteed.

During recent visits to local greenhouses, growers have been showing me vegetative cuttings with chlorotic and necrotic leaf margins on lower leaves (see photo). In most cases, these cuttings were delayed in shipment for at least 24 hours because of something related to the winter weather. The yellowing leaf tissue could be a symptom of exposure to increased levels of ethylene or an indication that the cuttings are starved for energy (sugars).

An ethylene primer for those who could use a refresher

Ethylene is a small, naturally-occurring organic gas. It's a common by-product of combustion, but it is also produced within most types of plant tissue. Plants use it as a signaling molecule and, because it's a gas, it can float around and serve as a signal from one plant to many others. Ethylene can induce a broad range of responses within plants, including:

• Breaking seed and bud dormancy
• Ripening of fruits
• Leaf epinasty (downward bending)
• Lateral cell expansion (stunted growth)
• Flower inhibition, induction or abortion
• Senescence (decaying tissue)
• Root and root hair formation
• Abscission (leaf drop)

The specific effect it will have on any particular plant is context-dependent and varies across species, varieties, development stages and even tissues. Nevertheless, it is very common to see an increase in leaf senescence and abscission from exposure to higher concentrations of ethylene. Physiological stress and physical wounding can trigger an increase in ethylene production within plants. Vegetative cuttings that are sitting in closed containers on transportation vehicles for several days may be experiencing a combination of drought and temperature stress in addition to already being wounded. Close quarters within the packaging may keep increased levels of ethylene in prolonged contact with the plants, exacerbating the issue.

These situations are not ideal, often unavoidable, and completely out of your control. Erik Runkle, professor of horticulture at Michigan State University provided the following thoughts on these situations.

"Once the plant is stressed, there’s not much that can be done; the stress has already happened,” said Runkle. “So, the best growers can do is provide the best environmental conditions for rapid rooting. Unpacking cuttings as soon as possible, getting them hydrated and then stuck in a propagation area with very high humidity and/or misting is the best way to go at this point."

With that in mind, consider the following suggestions for giving your stressed-out cuttings the best chance to recover and thrive.

Stick the cuttings and place them under the mist as soon as possible. Under ideal conditions, cuttings may be fine in the cooler for a day or two. When the shipment is delayed, hydration is priority No. 1. Monitor cuttings closely and check on their progress every hour or so.

Consider applying a horticultural surfactant or wetting agent. Surfactants reduce water tension and allow the water to spread across the plant surface more evenly. Conceptually, this should result in increased humidity across the whole plant and allow for quicker and more uniform absorption into plant tissue. There are several products labeled for use on cuttings and readily available on the market (e.g., Capsil, UpTake, Suffusion, etc.). For more information, contact a member of the Michigan State University Floriculture Team.

Document the incident and contact your supplier. Seed and cutting suppliers are strongly interested in end-product quality and will often provide support in a number of different ways. Upon arrival, open every box and inspect the cuttings for any obvious signs of damage (e.g., freezing, disease). Record the number of plants of each color and variety that don't survive.

Trim off the senescing leaves and apply a broad-spectrum fungicide or a light sanitizer. Consider these options once the plants hydrate and start rooting. Because the older leaves tend to decay and fall off, Botrytis is bound to show up to the party. Cleaning up the plants once they're stable will go a long way towards disease control.

Reference to commercial products or trade names does not imply endorsement by Michigan State University Extension or bias against those not mentioned.