Washing Seedling Roots Reduces Survival of Loblolly Pine

 William A. Carey, David B. South, Thomas A. Greene, and John R. Britt

In terms of acreage, loblolly pine is the number one crop in Alabama with more than 2.9 million acres of plantations. A major concern of tree farmers is obtaining seedlings that will have a high potential for survival after planting. A recent study by AAES researchers has shown that the practice of washing roots to remove soil lowered the potential for survival.

Across the South, more than 1.2 billion bare-root pine seedlings were planted in 1996. Typically, seedlings are mechanically harvested, packed in “bundles,” and then either trucked directly to the outplanting site or the bundles are placed in cold storage for a few weeks before use.

As seedlings are lifted from the seedbed, soil adheres to the roots and this can add significantly to the weight of the bundles. The amount of soil that remains on the roots will vary with soil texture and weather conditions. The extra weight can increase shipping costs and cause ergonomic problems. Therefore, some nursery managers have developed systems for washing harvested seedlings before packing. The benefits can be substantial, especially when seedlings are lifted from wet, fine textured soils. For example, one nursery reduced average bundle weight from 62 pounds for nonwashed seedlings to 35 pounds for washed seedlings. Over the three-month planting season, the nursery recovered 100 tons of soil from the waste-water. Over decades, the loss of topsoil can be substantial and the lowering of nursery fields can result in an adverse change in surface drainage patterns.

When industry foresters began to question if there might be a connection between reduced survival and root-washing, they contacted researchers with the Auburn University Southern Forest Nursery Management Cooperative. Initially, it was assumed that any reduction in survival might be attributable to other changes in cultural practices since previous studies demonstrated increased survival from soaking bundles of loblolly pine seedlings. However, tree planters in Australia and New Zealand are told that some soil should be retained on pine roots. Studies with conifers from other regions of North America indicate that washing roots might decrease survival (Table 1). In fact, researchers have reported a 43% decrease in survival from washing the roots of ponderosa pine. Therefore, a field trial was established in 1997 to compare the effects of washing on growth and survival of loblolly seedlings.

Bare-root seedlings were harvested, washed, and placed in storage on February 5 and 17. Seedlings to be washed were placed on a conveyor belt below a series of nozzles and either washed once (at approximately 5 psi), or four times (at about 10 psi). Control seedlings were treated operationally but were not washed. On each lifting date, five replicates of 100 seedlings from each treatment were bundled separately after being sprayed with a gel to prevent root drying, and then placed in a seedling cooler. The seedlings were stored for either three or 15 days. All seedlings were removed from storage and planted on February 20. The seedlings were planted by hand with a contract crew. The planting site was located in Marion County, Georgia, on very sandy soil (Troup series). A drought-prone site was selected because survival differences among treatments are increased when seedlings are exposed to moisture stress.

Seedlings from the two lifting dates differed in average weights. The beds lifted on February 17 produced slightly larger seedlings. Although weights of stems and of large and fine roots differed between the two beds, the wash treatments did not significantly reduce seedling dry weight or root mass.

Within a month of planting, visual differences between treatments became apparent. There were fewer seedlings that were growing (terminal buds were elongating) in both the stored seedlings and the washed seedlings. In addition, seed-lings with washed roots did not appear as vigorous. By June, survival of seedlings that were washed once and stored for two weeks was 13% less than seedlings in the control group (Table 2).

A comparison of loblolly pine seedlings six months after planting. Roots of seedling on left were washed prior to planing in February (control seedling on right).

Table 2. Effects of Root Washing and Storage Duration on Bud-break (March), Initial Growth (May), and Survival (June) of Loblolly Pine Seedlings

Wash	Days storage	Bud break	Growth	Survival
		Pct.	In.	Pct.
0	3	24	5.1	99
1	3	17	3.5	94
4	3	14	2.8	82
0	15	17	3.5	95
1	15	14	2.7	82
4	15	11	1.9	66

Initially, washing alone was not expected to reduce survival. If washing roots increases seedling stress, it was initially assumed that physical damage to the fine roots would explain a loss of vigor. However, no treatment differences in either lengths of fine roots or the weight of small or large roots were detected. This, combined with studies of other pine species (Table 1) where other researchers used more gentle washing techniques, cast doubt on the "root injury" hypothesis.

In a subsequent study, washing roots reduced new root growth (6.7 inches of new roots for unwashed seedlings compared with 2.3 inches of new roots for washed seedlings). Further studies are planned to determine the exact cause of the reductions in root growth and seedling survival. Researchers plan to test the idea that washing roots causes more harm than soaking roots in water. One hypothesis states that a reduction in new root growth is due to the removal of important growth stimulants.