Rooting Reminders

Dr. P. Allen Hammer

Mist propagation can be a challenge. Most cuttings are rooted during the very worst weather of the year. Growers have to root multiple species of plants and most often have limited ability to provide a lot of different zones for different plants; therefore, they are faced with making compromises among the differences in species. But even with these challenges, growers can minimize problems by paying attention and understanding the principles of propagation. Light, temperature, water and rooting medium all interact during the rooting of cuttings. Different plants have specific requirements and must be handled differently, but in this discussion I will avoid those specifics and focus only on the broad issue.

Nearly every greenhouse I visit applies too much water in propagation. The mist nozzles are often too coarse and mist is most often applied too frequently and/or too long at each mist interval. Only enough water should be applied to cuttings to prevent the wilting of the leaves. Mist should not wet the rooting medium until after roots are formed. Mist application should be adjusted to account for weather, particularly sunny and cloudy days. Time clocks control the mist, but growers must control the time clock by paying attention to both the cuttings and the weather.

Rooting medium plays a huge role in both rooting and water management. A rooting medium that holds a lot of water will cause slower rooting and is much more difficult for the grower to manage watering. Aeration is important for root formation, but growers can reduce the aeration of even the best rooting medium by applying too much water.

The temperature of the rooting medium is extremely important for rooting of cuttings. Rooting medium temperature must be monitored and controlled. And remember—controlling air temperature does not control rooting medium temperature. In fact, bottom heat is essential in most greenhouses, particularly during the cold winter months of propagation. Every greenhouse grower should have a digital thermometer in their pocket and use it often to check the rooting medium temperature. Also, avoid applying cold water to cuttings.

Sunlight also plays an important role in propagation of cuttings. Too often growers root cuttings under too low light during the winter months. Growers really need to understand Daily Light Integral (DLI) when rooting cuttings. As with temperature, light needs to be measured and controlled. Certainly not every greenhouse can add artificial photosynthetic light; however, every greenhouse can control the amount of sunlight on the greenhouse bench. Many months of the year, shading is probably not needed during propagation in many greenhouses.

It’s also extremely important to apply a liquid fertilizer solution to cuttings very early in propagation. As soon as roots initiate, a low rate of a complete fertilizer (75-100 ppm N) should be applied with the rate, quickly increasing to 200 ppm N as roots grow. The very best liners are produced when they receive adequate fertilizer. Growers are often reluctant to apply fertilizer to cuttings thinking the fertilizer will cause rank, too-soft growth. I find just the opposite, as lack of fertilizer cause soft, weak and stretched liners.

And certainly any discussion of propagation would not be complete without reviewing sanitation. Sanitation needs to be practiced every hour of every day, particularly in propagation. Greenhouse sanitation is most often related to how recent the grower experienced a disease problem. A disease problem elevates sanitation to its place of importance in propagation. The longer a greenhouse goes without experiencing a disease issue in propagation, the less importance is placed on sanitation and the sloppier the propagation. Sanitation in propagation should always be at the very top of growers’ concerns. Everything the cutting touches should be clean—including hands, benches and carts. Footbaths are not just important after a disease problem, they are important everyday in a propagation greenhouse. And always remember each cutting has a very large open wound until it calluses and roots.

I encourage growers to review their propagation techniques. All the seemingly little details make all the difference. Grower decisions can have a tremendous impact on the rooting of cuttings. GT

Dr. P. Allen Hammer is a retired professor of floriculture at Purdue University, West Lafayette, Indiana, and is now in product development and support for Dümmen USA.