Gravel-Planted Trees Can Survive Summer Heat, Save Labor

This method, called MGB for Missouri Gravel Bed, has been tested on 2-inch caliper trees and shows promise as a way for landscapers and nursery operators to save labor, equipment and fuel costs by handling plants bare root, rather than with a heavy ball of soil, said Starbuck.

"There are a lot of potential labor and transportation cost savings," explained Starbuck. He predicted that the freight costs for 2-inch caliper bare root trees would be a tenth of what it would be for B&B stock because he said nurseries could get 10 times more bare root trees on a semi trailer. "You could haul probably six to eight times as many trees to a landscape job with a single truck, get by with a less heavy-duty truck and need less equipment to haul trees around the job site," Starbuck continued. "If we find that MGB works as well for larger trees, such as 4- or 6-inch caliper, then it might open up a lot of possibilities for contractors."

Bare roots of 2-inch caliper ash tree.

MGB FEATURES. When using MGB, dormant trees are placed with their roots in a bed of frequently irrigated river rock. Starbuck's gravel bed is built on a slope with a pond liner underneath, allowing irrigation water to be collected in a sump. Water is then re-circulated by drip irrigation tubing attached to a time-clock-activated pump. The system, which he described as a low-tech hydroponic system, irrigates for three minutes every hour.

Starbuck said it is important to use appropriate materials in an MGB system to promote tree growth and successful transplantations. He suggested using river rock or pea gravel, ranging from 1/2 to 3/8 inches in particle size, mixed with about 10 percent sand. "[The sand] helps the rock hold a little more water, and with drip irrigation, it helps distribute water more laterally so you get more uniform wetting of the rock," he explained.

Because sand clogs pore space in the rock, Starbuck warned about using too much sand in a gravel bed. Additionally, tree roots growing in sand tend to spread widely, which could lead to more root damage when removing trees from the bed. "It’s not all that critical what rock you use, as long as there are enough fine particles in it to disperse the water and not too many fine particles that cause the roots to spread too much," Starbuck noted. "You also have to match the irrigation regime to type of rock you have."

Chris Starbuck explains his Missouri Gravel Bed method of tree production to attendees of a Turfgrass and Ornamental Field Day at University of Missouri.

MGB STUDIES. In a preliminary 2000 study, a 15-foot-tall ash tree was removed from gravel and planted in July. "Despite being watered only once two days after planting, the July-planted tree showed no sign of wilting or scorch and had normal sized leaves in 2001," noted Starbuck.

In a March 2001 follow-up project, researchers removed soil from the roots of six 2-inch ash trees and placed them in a gravel bed. Another six similar trees with their soil balls intact were kept moist by hand watering. In June 2001, the six MGB trees had slightly smaller leaves than the soil-balled trees but had extensive root growth in the gravel, according to Starbuck.

The 12 trees were then planted in the field on July 9 and 10, 2001, two of the hottest days of the summer, and none of the trees wilted after planting, despite the 95-degree temperature. Starbuck said the trees will be evaluated for two growing seasons for stem caliper and shoot growth to see if there is a difference between MGB and conventional trees.

Starbuck expects the two-year study to prove some hypotheses he has developed about the MGB method and using bare root trees compared to B&B trees. "I’m not saying that the bare root trees will always grow faster than B&B trees," he said, "but they do get outstanding survival because the roots you take out of the rock go right into contact with the soil where they’re going to be growing, and you don’t have any interface between the ball and the surrounding soil that’s likely to cause problems."

Common problems that can occur when planting B&B trees are related to conflicting soil types between a tree’s soil ball and the soil it is planted in. Starbuck explained that if a tree dug from a clay soil nursery is planted in a sandy soil and a contractor uses irrigation methods for sandy soils, the tree may drown because the clay soil ball will hold more water than the tree can handle. If the scenario is the opposite with a silt or loam soil ball planted into heavy clay, the tree may drown from pulling water out of the surrounding clay, or the tree may wilt if the clay is dry and it pulls water out of the soil ball. "Whenever you have a difference in soil type between the ball and the surrounding soil, there’s likely to be some water movement problem," Starbuck said. "But with the bare root tree, there is no interface. Once you pour a bucket of water in the hole after transplanting, the tree is on its own. The root tips are functional five minutes after the tree is planted."

IMPLICATIONS OF MGB. "I would guarantee [bare root trees’] survival before I would a ball or container tree just because the roots are functional from the moment they’re planted," said Starbuck. Therefore, contractors could benefit from the MGB method by reducing the amount of maintenance required after planting. "The cost per surviving tree is likely to be very favorable compared to B&B for a lot of applications," he noted.

Patmore ash ready to be wrapped in tarp for transporting.

Transporting bare root trees requires less room and adds less weight than transporting B&B trees, said Starbuck, explaining that it may take four people to move a 2-inch B&B tree compared to one person moving a 2-inch bare root tree.

Whereas B&B trees can be watered and remain out of the ground for longer periods of time, bare root trees should be planted shortly after removal from a gravel bed. "All you really have to do at this time of year when the leaves are hardened up is keep the roots from drying out," said Starbuck. He moves bare root trees by laying a tree on a tarp, spraying the roots with water and wrapping the tarp around the roots. As long as the trees are kept out of the sun, he said they can remain in the tarp for about four to five days. "They’re not nearly as perishable as most people think they are," Starbuck said.

Using bare root trees might add a little extra labor after planting. Unlike B&B trees, bare root trees must be staked because there is not a ball of soil offering stability. However, Starbuck said that depending on the quality of the soil, one season may be enough time for the trees to not need stakes.

Although transplant shock occurs whether digging trees for bare root or B&B transplantations, Starbuck explained that the MGB method is a way to help a tree get through the transplant shock. "I don’t really view [MGB] as a growing technique, especially if you get into larger trees," he said. "So I don’t think the wholesale grower is too likely to be interested in this, except if they start harvesting bare root and selling them to people with gravel beds. It would also help [growers] get around the problems associated with finding skilled labor to dig B&B trees because it takes a lot less knowledge and skill to dig a bare root tree than it does a B&B one."

Thus far, Starbuck said research on the MGB method has been slow because of the difficulties in finding large tree donors. He has received several 2-inch trees from a local nursery, but he is looking to test his method on 4- and 6-inch trees to determine if he is right about the potential for the MGB method to work on larger trees. He is optimistic about the implications MGB could bring to contractors and growers. "If we can get people trying it, it could catch on and find its place in the industry," he said.

Starbuck can reached via e-mail at StarbuckC@missouri.edu.