Don’t allow cuttings to become water stressed because stress increases the risk of crown budding. Start to establish wet/dry cycles, but don’t let cuttings or liners become dry or wilted. Photo by Sally Colby

by Sally Colby

Many who visit garden centers in late summer and early autumn are looking for mums to brighten the landscape, so mum growers strive for full, healthy plants. Nick Flax, Penn State Extension horticulture team, provided tips for successfully growing mums.

For those starting with unrooted cuttings, Flax said the most important aspect is to be prepared. “The last thing you want is to be caught off-guard and have unrooted cuttings come in and not be ready to get them stuck and out into the propagation area as soon as possible,” he said. “It’s important to maintain a dialogue with your unrooted cutting supplier. If there are any changes in shipping schedules, you want to know that, and be able to have your production team shift priorities to get unrooted cuttings taken care of as quickly as possible when they come through the door.”

The arrival of unrooted cuttings of any kind should take priority, and Flax advised growers to be prepared to stick them immediately or store them in a suitable area to maintain maximum quality until they can be stuck.

When cuttings arrive, open the packaging and inspect the cuttings. Growers who don’t inspect for signs of disease may be unpleasantly surprised to see botrytis or other issues days or weeks later. “Open the pouch, move the cuttings around a little and give them a thorough inspection,” he said. “Look for excessive wilting – these are tender, herbaceous plants so they’re going to lose a little turgidity from the time they’re snipped off the mother plant at a stock plant farm. If cuttings are excessively wilted, you’re going to have limited success getting them to root uniformly.”

Pest-damaged cuttings are more difficult to establish and can be an infection route for pathogens. Check for pests – thrips are notorious for hiding on mum cuttings. Flax said while cutting farms do their best to manage pest pressure, thrips are everywhere, and if they’re present, cuttings should be treated with an appropriate dip.

It’s critical to maintain the cold chain from the time cuttings are cut until they’re planted in the greenhouse. A cooler with a cold fog system will help rehydrate cuttings that must be stored. The environment cuttings are stored in should be at least 85% relative humidity with temperature between 35º – 40º F. Flax recommended that in systems with vapor pressure deficit control, the VPD should be below 0.3 kilopascals.

Growers rooting their own liners should have liner trays filled and ready to go. Flax said it’s difficult to manage the logistics of having some of the crew setting plants on benches while others are sticking cuttings.

“Soil should be clean, loose, coarse to medium,” said Flax. “You don’t want to use a fine germination mix. If you’re using intermittent mist, overhead mist or fog systems, the media can get saturated quickly, so you need good drainage with sufficient moisture retention.” There should be uniform moisture across the leaf surface, but if the medium is soaked, root development will be inhibited because the adventitious roots are deprived of oxygen.

Stages 1 and 2 in mum production are sticking and callusing, and it’s important to begin by sticking cuttings at the appropriate depth. “If you’re burying lower leaves, it predisposes the cuttings to disease,” he said. “It’s okay to have some leaves resting on top of the substrate, but don’t bury the leaves. If you’re doing direct stick into smaller containers (four- to six-inch pots), have them filled in their shuttle trays out on the bench and make sure the substrate isn’t too dry.” Overly wet cuttings take longer to bounce back.

Trays should be moved out of the propagation area and into high humidity as soon as possible. It’s easy to stack up liner trays, put them on a cart and push them into the greenhouse, but there’s potential for losing cutting quality if they sit on a cart and dry out.

Rooting hormone is always a good measure for mum production. Flax said mums are easy to root, but using IBA (a synthetic auxin rooting hormone), whether it’s a solution or a powder, will increase the rate of adventitious root formation and will increase uniformity. “In floriculture, we cultivate aesthetics, and those aesthetics are contingent on crop uniformity,” he said. “If you’re doing a basal dip in a solution of between 500 to 1,000 ppm IBA, that’s a suitable range.” Cuttings can be dipped in 1,000 ppm powder, tapped gently to removed excess power and stuck in the liner tray.

“You can also do foliar application,” said Flax. “After you’ve stuck the cuttings in the tray and put them in the propagation area, come back and apply a solution of IBA over the entire crop. This works well if you add a surfactant to help spread the IBA so it’s more easily absorbed.”

At this point, light level should be low. “Keep DLI of about 5,” said Flax. “You don’t want cuttings exposed to high light conditions – that’s going to increase the risk of desiccation.” Shade cloth should be pulled over cuttings on bright, hot days.

In stage 3, adventitious roots start to develop. “Through stages 1 and 2, down to stage 3, you’re gradually decreasing relative humidity and increasing the interval between mist or fog application,” said Flax. “The general rule is to decrease relative humidity by a few percent each day throughout propagation. Increase VPD to 0.5 or 0.7 kilopascals. As the callus tissue forms and as adventitious roots start to grow, you’re encouraging roots to grow even further and reach more into the propagation media to start pulling water up and supplying the leaf canopy with water rather than absorbing it through the leaf surface.” Water moving through the developing root system is critical for good root development – without sufficient roots, media falls off at transplanting.

Mums are a heavy feeding crop and sensitive to nutrient stress, so at this point, start applying fertilizer solution. “In the propagation environment, they’re constantly getting soaked, there’s lots of moisture and inevitably stuff is going to leach out of medium if you have a starter charge in that medium,” said Flax. “Hungry cuttings you will increase the risk of crown budding, so start feeding relatively early once you start to see adventitious roots.” Begin increasing the daily light integral, and over time, increase from 5 to 10 moles/day.

By stage 4, eliminate misting in the main production area. Relative humidity should be close to ambient humidity, especially if mums will be moved outside after transplanting. Plants need to be tough enough and hardened for potential harsh conditions. Increase VPD to 0.8 – 0.95 kilopascals, which is the level for good growth for most species. “Increase fertilizer concentration,” said Flax. “Mums are hungry plants. Ramp up BLI – 10 to 12 moles is ideal. Maximize the amount of sunlight.”

Flax cautioned growers to not allow cuttings to become water stressed because stress increases the risk of crown budding. Start to establish wet/dry cycles, but don’t let cuttings or liners become dry or wilted. “Make sure air temperature does not drop below 70º, especially once you start toning,” he said. “The flowering response is controlled predominantly by daylength, but temperature can also induce cuttings to start a flowering response.” Lighting (LED, HID or incandescent) for 16 hours is a good insurance policy and can help ensure plants are under long day conditions to prevent cuttings from trying to flower before they’re transplanted.

Once mums are transplanted and into finish production, they require high light, with shade cloth used only to reduce ambient temperature. “If you’re growing mums outdoors on a mum pad, gravel, asphalt or weed fabric,” said Flax, “you’re at the mercy of the elements. Light levels outside are higher than in the greenhouse.”