How to hold and pretreat cannabis plants before transplanting them

planning ahead cultivation

(This is the fourth installment in a series focused on cultivation planning for marijuana and hemp growers. The previous installment is available here.)

Conditions aren’t always ideal for marijuana and hemp growers to transplant young plants.

Inclement weather, delayed labor or transplanting equipment and a host of other delays can leave cultivators facing a harrowing wait to get tender transplants in the ground.

But this farming problem isn’t unique to cannabis. Marijuana and hemp growers can look to proven strategies to keep the young plants healthy until conditions allow for a successful crop.

Holding plants

Young plants can typically be held for up to 10 days without much issue or damage.

Any longer, and it can get tricky – growers might need to think about applying supplemental nutrients.

But it’s easy to overdo it with fertilizer, so Jeremy Klettke, founder and CEO of Davis Farms of Oregon, a hemp farm and genetics provider in Bend, advises holding off or applying a bio-inoculant instead.

“That’ll get the plants moving without causing shock,” he said. “And they’re coming with a bunch of their own nutrients already to begin with. So they’re loaded with nutrition out of the gate, and it’s just so easy to burn them.

“Generally speaking, we tell farmers (to) run a pretty inert media … and then don’t really add anything to it for the first 10 to 14 days.”

It’s important to keep young plants watered so they don’t get stressed, Klettke said. But it can be difficult to keep the water level just right.

“The difference between soaked and bone-dry is quick – it’s sometimes only a few hours – so you have to be really careful and cautious about that,” Klettke said.

Growers might need to bring in water to ensure the proper pH. Well water might be off balance, said Hamilton Nelson, sales and operations manager with Square One Starts in Aurora, Oregon.

One of the biggest concerns about holding young plants for more than 10 days is the danger of the plants getting rootbound “like a noose,” Klettke said.

“You really only have maybe a 10-day window, maybe 14 on the outside, to get those plants in the ground from the time that they’re ready until the time that they’re just too mature to safely go in the ground and give you a good profit.”

At that point, growers might need to consider whether they should cut their losses and ask the young-plants provider to start rooting new clones or seedlings.

Requesting young plants in larger-sized 72-cell trays can often alleviate root circling, Nelson said.

“You can hold a 72-deep a pretty long time, without too much worry,” he said.

“I think some will look at the efficiency of how many plants are in a 128 tray versus the larger 72. But what they really should be looking at is what type of soil are you going into.

“If you’re going into extremely moist, wet soil, a 128 is just fine. But if you’re going into sandy, dry soil, which most people are, a 72 is a much better product.”

Pretreating before planting

Growers should make sure they are starting with the cleanest plants available. The greenhouse should provide a certificate that confirms their shipment is clean.

However, it’s smart to assume that young plants, especially clones, are infested with pests from mites and whiteflies to diseases such as hop latent virus and treat accordingly, said Michael Brownbridge, biological program manager, disease control management with Bioworks and a former research director for horticultural production systems at Vineland Research and Innovation Center in Ontario, Canada.

“You name it, the threats come in,” Brownbridge said.

“The mites are really hard to see, and if you don’t get a handle on them straightaway, you’re playing catch-up and good luck, you may never catch up with some of these.”

Hop latent virus “has everybody freaked out,” he said, because plants can be infected without showing symptoms.

“Take a selection of (young plants) from every batch and send them out for independent testing before planting them up, to make sure that they were disease-free,” Brownbridge said.

A few controls are available for cannabis plants such as dipping clones into pest-control products before planting – a technique common in ornamentals production – that are in the application process for marijuana and hemp in Canada and the U.S., according to Brownbridge.

“I would get on a preventative spray immediately with something like SuffOil to knock back the mite population and get a (biocontrols) program in place immediately,” Brownbridge said.

If the young plants are being held indoors for any length of time before being transplanted outdoors, predatory mites will control pests like two-spotted spider mites and broad mites, although they don’t work well against russet mites, which is where the oil comes in, Brownbridge said.

Root aphids are another problematic pest outdoors with few options for control.

Brownbridge said a fungal drench is the only product approved on cannabis crops currently, though that differs from state to state.

“Azadirachtin combined with a fungi will do a good job against root aphids, but, again, it depends … if that material is approved in that particular state.”

For disease control, Brownbridge recommends asking the young-plant propagator to apply an inoculant on the roots, “be it a Trichoderma or Bacillus.”

“Trichoderma grows better for longer on the roots than Bacillus do,” he said, “but have something on there and continue that, because having it on the roots and then planting out, your seedlings are sort of protected as they go out into the field – and if you can get another dose on just as they’re planted out in the field, even better.

“That’s something I would highly recommend to give those plants the best start once they arrive and give them that good start once they go out into the field.”

Laura Drotleff can be reached at [email protected].