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Hannah Mathers, The Ohio State University, Horticulture and Crop Science |
Soil temperature is a very important factor in the growing of woody plants. A plant cannot be any hardier than its roots. If the roots are unable to endure the temperatures to which they are subjected, the plant cannot survive no matter the hardiness of the above-ground plant parts (Patterson, 1936). Many studies of root cold hardiness of numerous woody plants have demonstrated that the root system is substantially less hardy than that of the shoot system (Weist and Steponkus, 1977). Much of the death above ground that occurs in woody plants, especially in containers, is doubtless the result of root death and of the destruction of many of the absorbing organs of the plant.
Mityga and Lanphear (1971) proposed that only mature roots could harden off to freezing temperatures because the high levels of gibberellic acid produced in root tips of the young roots nullified the effect of the growth regulator. Root cold hardiness studies generally indicate that mature root hardiness values are several degrees lower than for the young roots. In many cases, mature roots may survive even when young roots are killed. This explains, however, why some plants flush much later than normal in the spring, show retarded growth throughout the growing season, and may become susceptible to root rot or other disease pathogens. Overwintering practices need to be sufficient to protect mature and young roots from injury. The influence of root maturity has not been studied for high temperature stress.
There are many things that need to be researched in the area of temperature disorders and ornamental culture. However, good container media and nutrition management is basic to the production of quality container-grown plants (Davidson etal., 1988). To minimize environmental impact, many growers are using controlled-release fertilizers (CRFs) and composted materials. High levels of nitrogen have been shown to inhibit both dormancy and cold hardiness. Nielsen (1974) indicated adjustments in amount of water and nutrients supplied could partially ameliorate the affects of unfavorable root-zone temperature. However, Yeager etal. (1991) showed a threshold point was passed at 103F. At temperatures above 103F, nitrogen accumulation decreased regardless of nitrogen application rate. Although research indicates that supplemental fertilizing with water-soluble fertilizers, particularly nitrogen, is beneficial for fast-growing crops when using CRFs and that composts are good soil amendments for physical properties in containers, the effects of fertilizer regimes on dormancy induction, cold hardiness of roots and shoots, and high temperature tolerance have not been sufficiently examined.