Fast and ferocious

A never-ending propagation challenge is weeds.

In a presentation hosted by the Horticultural Research Institute (HRI), research associate professor Anthony Witcher, Ph.D., Otis L. Floyd Nursery Research Center at Tennessee State, discussed weed control in propagation.

“We think propagation is a short-term process,” said Witcher. “We grow seeds or cuttings for a few weeks then sell them. But these liners often sit for months. Weeds are good at what they do. They’ve evolved over the years to grow as fast as they can, flower quickly and produce as many seeds as possible. Weeds grow fast and spread ferociously, and that’s why they’re so difficult to control.”

Witcher noted that bittercress can go from seed to flower and seed production in just six weeks, producing up to 5,000 seeds per plant. A crabgrass plant can produce up to 150,000 seeds over the growing season. Seeds are efficient at dispersing through wind, water, animals and explosive dehiscence – the result of internal pressure that can send seeds several yards away.

Weeds come from different sources, including container substrate. Piles of pine bark stored outdoors will pick up numerous weed seeds via wind. Seeds are often present in reused containers; seeds from certain plants, such as bittercress and oxalis, have a sticky seedcoat and easily attach inside containers.

Greenhouse floors, especially gravel floors, and even fabric, are hotbeds for weeds. If potting mix falls onto the floor, weed seeds combine with the mix. Grass and drainage ditches surrounding greenhouses harbor weed seeds. Workers also unwittingly collect and disperse weed seeds.

“Sanitation is the first line of defense for preventing weeds in any kind of propagation,” Witcher said. “The more we do to keep things clean, the fewer weeds we’ll have.”

He advised growers to do whatever is necessary to keep weed seeds out of liners. Careful substrate storage can help limit chemical use for weed control. Concrete pads and walls between bins of different substrates help keep seeds out.

As weeds grow, especially before they flower, remove them. Avoid the temptation to reuse substrates. It’s hard to waste material from a lost crop, but Witcher said there might be a good reason the crop failed, such as the presence of soilborne pathogens.

“If you want to reuse substrate, sterilize it through steam, which isn’t really cost effective, or have a good composting system with temperatures up to 140º to kill weed seeds,” he said. “If you reuse containers, pressure wash or steam clean them to remove weed seeds. Do a good job controlling weeds in surrounding areas.”

Although sanitation is a good start, weeds break through and other practices are necessary. “The first step is managing weeds in the floor of the propagation area,” said Witcher. “Some post-emergent herbicides can be used for that. These products are non-selective, so be careful when applying around the crop so spray or drift doesn’t get on the leaves of the crop. Most are labeled for inside, enclosed structures like a greenhouse, so use good ventilation.”

Broad-spectrum pre-emergent herbicides such as indaziflam and flumioxazin can help prevent weeds. Ideally, these are applied prior to moving flats or containers inside the propagation area, which allows the entire area to be treated. Treat floors, wait several weeks and air the greenhouse prior to moving plants in. Witcher noted that these chemicals are not labeled for use in greenhouses when plants are present.

For those who prefer to avoid chemicals completely, Witcher suggested mulches. Rice hulls are commonly used in production and work well. Other materials such as pelleted products that show promise include paper pellets (recycled paper debris) and pine pellets (compressed sawdust), which are typically used for small animal bedding. These materials become saturated and expand, creating an effective weed barrier.

With a propagation mix, the goal is a balance of air space and water retention to ensure good root development. “If you put something on top of that that has different physical properties that may affect rooting, a big chunk of the cutting is going to be stuck in the mulch,” Witcher said. “We don’t really know how it’s going to affect rooting.”

Mulch trials at depths of a half-inch and one inch showed that some mulched cuttings didn’t respond well in containers, while others weren’t affected. “It depends on the crop,” he said, “and maybe the size of the cutting and the propagation system as to how mulches affect rooting.” Witcher suggested propagators test various mulches and cuttings to see which work best.

Pine pellets and some of the herbicides performed well in inhibiting weed growth. Although rice hulls weren’t in Witcher’s trial, he said they work well in production because they are naturally hydrophobic and stay dry.

“They keep the top layer dry so weed seeds can’t germinate,” said Witcher. “In propagation, we’re applying a lot of water, so it keeps the rice hull layer more moist. Seeds can germinate and roots make their way down to the potting mix where the fertilizer is.”

When Witcher trialed pine and paper pellets, weeds germinated but roots couldn’t penetrate the mix and reach the fertilizer. The physical barrier kept roots from growing down into the pot, but there was a phytotoxic effect from chemicals in the sawdust.

“We collected leachates from the mulches to see if there was anything in the material that had an effect on germination,” said Witcher. “Pine pellets were the only ones that had something that inhibits seed germination. That might be why we saw stunting in the seedling trial, but pine pellets do a good job with weed control.”

Mulch depth affects weed control. With bittercress, pine and paper pellets at a half-inch provided excellent control. Rice hulls at one inch did a better job controlling bittercress but weren’t as effective as pine or paper pellets. Pine and paper pellets at a half-inch and one inch worked well for controlling crabgrass.

Because pine pellets expand, Witcher advised placing one layer at the top and letting them expand to fill the rest of the pot.

For seedling propagation using pre-emergent herbicides, test individual species in a small area to ensure herbicide safety.

“Mulches such as pine and paper pellets look like a great alternative to pre-emergent herbicides,” said Witcher. “Apply these at least a half-inch deep, but no more than one inch to avoid rooting issues.”

by Sally Colby