Southern bark substrate (left) vs. northern bark (right). The size difference doesn’t really matter – it’s the bark’s water holding capacity that’s important. Photo by Courtney Llewellyn

by Courtney Llewellyn

There are a lot of options out there when it comes to substrates: soil, first and foremost, but also sphagnum peat moss, coir, perlite, vermiculite and bark. Luckily, growers in the U.S. have an enormous supply of bark coming from the Southeast – or, as one researcher calls it, “the wood basket of the world.”

Brian Jackson, Ph.D., professor and director of the Horticultural Substrates Laboratory at North Carolina State University, lectured on the proper handling, storage and utilization of bark substrates at this year’s Cultivate Expo. He said bark is popular because of its versatility, but those who use the medium need a bark management plan in which everything is written down. Solid plans are needed because biology is always changing and therefore there is always a moving target for product quality.

The processing and handling of bark starts with “barkitecture,” according to Jackson – creating specific products for specific needs. (He highly recommended visiting a lumberyard to see bark being processed and refined into nuggets, mini nuggets and substrate in person, if possible.)

Once it’s sized properly, handling it is like grilling, Jackson said. “It’s about managing the heat,” he explained. That can be done with thermometers or infrared cameras. It can’t get too hot. “Pay attention to color changes too,” he added. Fresh bark will be orange/tan; aged bark will be darker brown/black.

When it comes to growers as bark managers, Jackson said there will hopefully be someone on site to see the bark as it’s being delivered to make sure it looks right. “Keep a reference sample,” he said. “Check for moisture content and smell. Check the pH and EC when it arrives and every two to three weeks before use.”

Jackson also said to keep equipment off stockpiles (even a lawn mower can cause compaction). If possible, store the bark on concrete pads; keep weeds away; routinely check the temperature; mix it on the concrete; and most importantly, keep the inventory from drying out.

Of course, like any other growing medium, there are problems to be expected with bark, but Jackson outlined the ways to mitigate them. Management of large supplies is critical, as excessively dry and/or anaerobic conditions can develop, creating problems during and after potting. Inventories need to be turned and mixed, otherwise a gray band of mycelium can develop about 24 inches below the surface (due to heat and evaporation). This band inhibits air exchange in the interior of the pile.

“Mycelium grows in very dry bark – below 35% moisture,” Jackson said. “If mycelium is present when bark is delivered, it may not be ready to use. The fungi don’t hurt plants, but the lack of moisture they cause does.”

If it ever looks like there is a haze coming from a pile of bark, it is most likely fungal spores, not steam. “If the bark is too dry or hazy, do not pot in it!” Jackson warned. “But, if you do pot, keep it wet. Percolate it. Consider repotting.”

Inventories also need to be turned to keep bark from becoming too hot. If the bark arrived hot or has been undisturbed for two to three weeks, test its pH and EC. To do so, fill three or four pots with bark samples, then irrigate the pots and allow them to drain. After 20 minutes, pour a half cup of water over each pot and collect the leachate. The bark is good to use if the pH is higher than 3.8 and the EC is less than 0.5 mmhos. Make sure to regularly turn, flush and cool bark to reduce its heat. Keep it moist with sprinklers.

If the bark is pyrolytic (meaning it looks burnt), Jackson said to simply mix it well with good stock. It’s still usable. If it dries completely, however, “it becomes hydrophobic and it will not become damp ever again,” he said.

Bark age can also be an issue, whether it’s fresh, aged or composted. Fresh bark will be lighter in weight, hold less water, can be difficult to wet or re-wet and has a higher pH. Jackson said fresh bark is used more often in mixes in northern operations, but lower percentages are used and they’re often combined with peat and compost. Most bark is aged – but the older it is, the more likely it is to have contaminants, especially sand.

Another worry with fresh bark is white wood content. “White wood is not bad!” Jackson said. A survey of 13 bark products found between 4.5% and 22% white wood present, and further research showed the wood had no effect on fertility. “It’s larger in size, with a different porosity,” Jackson explained. Having less white wood just increases the mix’s water holding capacity.

An issue that’s popped up more in the last 18 months has been supply chain interruptions. “Based on growing media demand and shortages in the future, consider ordering early and/or creating suitable bark storage areas on-site,” Jackson said.

Those interested in learning more about using bark substrates can see Jackson’s research at