Ornamental Plants
Annual Reports and Research Reviews 2002

Special Circular 189

Growth and Water Use by Four Leguminous Tree Species in Containers on a Gravel Surface or Embedded in Mulch

Michael Knee, Daniel K. Struve, Michael H. Bridgewater, and Joseph W. Phillips, Department of Horticulture and Crop Science, The Ohio State University.

Results

The temperature of the medium in the containers was measured on three sunny afternoons when average air temperature was 30.3°C. There was a slight but significant difference (p = 0.01) between pots in mulch (25.4°C) and gravel (26.0°C). Containers of medium without trees lost more water (0.261 kg) on gravel than those in mulch (0.205 kg). The difference was significant (p = 0.001), and the loss from a container in mulch was equivalent to the average recorded evapotranspiration for the 5 days of measurement (4.36 mm).

Robinia pseudoacacia (black locust) was the fastest-growing species, reaching 2 m in height by the second harvest in July (Figure 1). Cercis canadensis (redbud) and Gleditsia triacanthos (honeylocust) reached a similar height by the third harvest in October, but Gymnocladus dioicus (Kentucky coffee tree) did not exceed 0.5 m. R. pseudoacacia trees were slightly taller when grown in mulch than on gravel, whereas the opposite was true for G. triacanthos (Figure 1).

Figure 1. Average height of trees during production in containers on gravel (open points) and in mulch (solid points): R. pseudoacacia, G. triacanthos, G. dioicus, C. canadensis.

R. pseudoacacia developed a root system that filled the containers with a dense mat of fibrous roots by the second harvest. Roots of C. canadensis approached a similar density by the third harvest, whereas the roots of the two other species did not completely fill the containers. The dry weights of roots were higher for containers in mulch than on gravel for all species by the third harvest (Figure 2).

Figure 2. Average dry weight of roots of trees during production in containers on gravel (open points) and in mulch (solid points): R. pseudoacacia, G. triacanthos, G. dioicus, C.canadensis.

R. pseudoacacia produced much higher shoot dry weight than the other species, mainly because of its massive stem development (Figure 3). R. pseudoacacia in mulch had higher shoot dry weight than on gravel, particularly at the second harvest, whereas G.triacanthos had higher shoot dry weight on gravel (Figure 3).

Figure 3. Average dry weight of shoots of trees during production in containers on gravel (open points) and in mulch (solid points): R. pseudoacacia, G. triacanthos, G. dioicus, C.canadensis.

Leaf areas were also highest in R.pseudoacacia except at the end of the experiment when leaf fall was occurring in this species, and at this time C. canadensis had developed more leaf area than the other species (Figure 4). Leaf areas were similar for trees in mulch and on gravel.

Figure 4. Average leaf area of trees during production in containers on gravel (open points) and in mulch (solid points): R. pseudoacacia, G. triacanthos, G. dioicus, C.canadensis.

Water consumption was estimated from the change in weight of containers, recorded on three successive days three times during the growing season. We hoped to be able to discriminate between water consumption by the tree itself and in the container by comparing weight changes for foil-covered and uncovered containers. However, inspection of the data revealed that the foil cover was not nearly so effective when applied around a tree stem as for a pot of medium without a tree.

As an alternative, the slope of the regression of weight change on leaf area was used to estimate tree water consumption (Figure 5). Water consumption appeared to be lower for C. canadensis and G. dioicus than for G.triacanthos and R. pseudoacacia. Water consumption for trees in mulch was higher than on gravel for R. pseudoacacia and C.canadensis.

Figure 5. Water loss in 24 h by trees in containers on gravel (open points) and in mulch (solid points). The points represent the range of leaf areas recorded and the rates of water loss calculated from the regression of water loss on leaf area. Average potential evapotranspiration was 4.35 mm day-1. R. pseudoacacia, G. triacanthos, G. dioicus, C.canadensis.

After planting out, G. triacanthos and R.pseudoacacia showed the largest increase in height, although G. dioicus showed the largest percentage increase (Table 2). G.dioicus and R. pseudoacacia from containers in mulch remained taller than those from gravel at the end of the first season in the field, but C. canadensis and G. triacanthos from gravel were taller than from mulch (Table 2).

Table 2. Average Height of Trees at Time of Transplanting (4/4/02) and After One Season's Growth (10/8/02).
Species	Production	Height (m)
		4/4/02	10/8/02
Cercis canadensis	gravel	1.66	2.03
	mulch	1.51	1.80
Gleditsia triacanthos	gravel	1.90	2.46
	mulch	1.58	2.38
Gymnocladus dioicus	gravel	0.31	0.70
	mulch	0.42	0.94
Robinia pseudocacia	gravel	1.40	2.38
	mulch	1.76	2.64