The Missouri Gravel Bed:

A System Allowing Planting of Bare Root Trees and Shrubs
12 Months of the Year

The Missouri Gravel Bed (MGB) is a method of handling bare root nursery stock in which dormant plants are placed in the spring with their roots in an irrigated bed of gravel and held for up to a year before planting bare root (in full leaf) in the landscape. It is not a growing method, but can be used by growers, retail and landscape nurseries, arborists and community forestry programs to extend the planting season and to greatly simplify the handling of bare root stock. It also shows promise as a method for heeling in B&B stock. The key to MGB is that root growth in gravel is very fibrous and, unlike with bark mulch, sawdust or sand, few roots are damaged when plants are removed from the gravel. The MGB was hatched at the University of Missouri Horticulture and Agroforestry Research Center about 1985. In the initial test, 20 of 20 4-6 foot bare root bush Washington hawthorn trees survived after field planting in mid-summer, after 8 weeks with their roots in aerated water. While aerated water grows good roots, it does not provide any support, creating problems in plant handling. Thus, studies since 1986 have used irrigated, creek gravel as a root growth medium.

Over the past 20 years, many species have been tested in the MGB including ornamental pears, redbuds, flowering dogwood, lindens, maples, crabapples and roses. A test in 1994 at Sherman Nursery, Charles City IA, expanded the number of species tested to well over 30, including some oaks and ashes up to 1 1/2-inch caliper. Sjulin nursery in Hamburg IA, has evaluated the performance of over 100 species of shrubs in the system. Overall, the field survival of MGB plants has been as good or better than that expected for container grown or B&B materials. There is nothing high tech about MGB. All that is required is a layer of gravel 14-18 inches deep and a time-clock-controlled irrigation system. Plants are simply placed, while still dormant in spring, with their roots in the gravel and allowed to grow until time to plant. A surface application of slow release fertilizer has proven effective in keeping the plants green. Under Missouri conditions, most species have done well using a drip system set to come on for 3-5 minutes every 3 or 4 hours between 8 AM and 6 PM even during hot, dry periods in July and August. Once the top growth has hardened a bit, the watering frequency can be cut back to perhaps twice a day. The irrigation time clock should, of course, be adjusted occasionally based on plant appearance. An irrigation kit is available from Craig Pisarkiewicz of MPR Supply, St. Louis MO including a time clock and solenoid valve and all of the required irrigation tubing and fittings. Recent research indicates that incorporation of 15 to 20% calcined clay into the gravel eliminates the need for recirculation and allows plants to be irrigated once per day with minimal stress.

The main reason why MGB works so well is that actively growing root tips are placed in direct contact with the soil in which they will grow in the landscape (no interface problems as with container and B&B stock). When plants are removed from the gravel, spraying the roots with water and putting them in a plastic bag is sufficient if the plants will be planted within a day or two. A bucket of water poured in the hole at planting time is often the only care required once the plants are in the ground. Aftercare required is generally much less than for container and B&B plants. MGB seems well suited for urban tree planting projects. Bare root trees are less expensive, cheaper to ship and easier to handle than B&B. And, since the system allows bare root trees to be planted all summer with survival rates comparable to B&B and container grown plants, labor scheduling can be much more flexible. In a recent study, 2-inch caliper green ash trees showed 100% survival when field planted bare root, in full leaf from a gravel bed in mid July. There was no difference in growth the year following planting between these trees and balled and burlapped trees of the same size planted in mid summer for comparison.

For More information, contact Chris Starbuck, 1-31 Ag Bldg, MU, Columbia MO 65211