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Brent Black, James Frisby, Kimberly Lewers, Fumiomi Takeda, and Chad Finn

factor for cane growth because it was correlated with heat unit accumulation (base 5 °C) throughout the season. Under temperate-zone growing conditions, floricane-fruiting blackberries have a clearly defined seasonal pattern of dormancy, entering the

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J.D. Carlson and J.F. Hancock Jr.

Abbreviations: THIGH, high-temperature threshold; TLOW, low-temperature threshold; SDATE, starting date for heat-unit accumulations. 1 Assistant Professor. Present address: Dept. of Agricultural Engineering, Oklahoma State Univ., Stillwater, OK

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Bill B. Dean

Asparagus is a unique crop in that the yield of the crop is entirely dependent on the storage of carbohydrates from the previous season. The number of spears produced is determined by the number of buds on underground rhizomes and the size of each spear is related to the size of the bud from which it originates. Growth of spears begins in the spring when some minimal temperature is reached in the soil. We have determined that the minimum temperature for spear growth is 10°C (50°F) and that temperatures in excess of 35°C (95°F) inhibit growth. Using data from growth response to temperature experiments, we have compared the accumulative effects of hourly temperatures preceding harvest to subsequent yields. There was a significant effect of the number of hours above 10°C for the 24 hours preceding harvest and the yield obtained. Yield of spears cycled over an 11-day period which correlated to an eleven day weather cycle determined from the heat unit accumulations. Heat unit and yield cycles for seven cultivars over a 4-year period will be discussed.

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Paul W. Bosland, Danise Coon, and Gregory Reeves

level, 1,207,764 SHU ( Table 1 ). ‘Trinidad Scorpion’, ‘Trinidad 7-Pot Jonah’, and ‘Douglah Trinidad Chocolate’ also had high average heat levels of 1,029,271; 1,066,882; and 1,169,058 SHU, respectively. Table 1. The mean heat level in Scoville heat

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L-Y. Li, J.H. Lieth, R.H. Merritt, and H.C. Kohl

A heat-unit model was established for tracking the development of geranium, based on experimental data collected at UC Davis and Rutgers Univ. The temperature thresholds for initiating development and heat-unit benchmarks needed to accomplish each phenostage are parameters in this model. The methods of estimating these parameters were proposed and tested with the observed data. The model worked well during either vegetative or reproductive stages, but failed to predict the initiation of flowers, suggesting that factors other than only temperature drive the flower initiation process. With this model crop development characterized by a series of specific morphological events can be tracked and predicted under various temperature regimes, so that crop timing can be more precise.

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Katharine B. Perry and Todd C. Wehner

A heat unit model developed in a previous study was compared to the standard method (average number of days to harvest) for ability to predict harvest date in cucumber (Cucumis sativus L.). Processing and fresh-market cucumbers were evaluated in 3 years (1984 through 1986), three seasons (spring, summer, and fall), and three North Carolina locations. The model predicted harvest date significantly better than the standard method for processing, but not for fresh-market cucumbers.

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Katharine B. Perry and Todd C. Wehner

The use of a previously developed model for predicting harvest date in cucumber production systems is described. In previous research we developed a new method using daily maximum temperatures in heat units to predict cucumber harvest dates. This method sums, from planting to harvest, the daily maximum minus a base temperature of 60F (15.5 C), but if the maximum is >90F (32C) it is replaced by 90F minus the difference between the maximum and 90F. This method was more accurate than counting days to harvest in predicting cucumber harvest in North Carolina, even when harvest was predicted using 5 years of experience for a particular location and planting date.

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Johnie R. Schmidt and Josiah W. Worthington

Contrasting colors of plastic mulch (black and white over black) were used to modify the rate at which heat units (HU) were accumulated in four different microclimates surrounding watermelon plants during 1996 at the Texas Agricultural Experiment Station-Stephenville. Daily maximum and minimum temperatures from 25 Mar. through 4 Aug. were recorded for air 10 cm above the mulch surface, at the mulch surface, at the soil surface under mulch, and 10 cm below the soil surface under mulch. Accumulated HU were significantly higher for white than for black mulch during two of the four periods monitored; however, the reverse was true for all other points of measurements at all times. Daily mean soil surface heat gain was 3.29 HU higher under black than under white mulch in early season, 6.21 higher in late April and early May, 5.19 higher in late May and June, and 4.19 higher in late June through July. Values for soil at 10-cm depth paralleled those for soil surface.

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Sylvie Jenni, Daniel C. Cloutier, Gaétan Bourgeois, and Katrine A. Stewart

Growth of `Earligold' muskmelon (Cucumis melo L.), expressed as plant dry weight from transplanting to anthesis, could be predicted using a multiple linear regression based on air and soil temperatures for 11 mulch and rowcover combinations. The two independent variables of the regression model consisted of a heat unit formula for air temperatures, with a base temperature of 14C and a maximum reduced threshold of 40C, and a standard growing-degree day formula for soil temperatures with a base temperature of 12C. Based on 2 years of data, 86.5% of the variation in the dry weight (on a log scale) could be predicted with this model. The base temperature for predicting developmental time to anthesis of perfect flowers was established at 6.8C and the thermal time ranged between 335 and 391 degree days in the 2 years of the experiment.

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Robert J. Dufault

The objective of this research was to determine the least variable method to predict the dates of the first and last broccoli (Brassica oleracea L. var Italica) harvests based on heat unit summation using coefficients of variation (cv). The method with the lowest cv for predicting first harvest was to sum, over days from planting to harvest, the difference between the growing season mean (GSM) temperature and a base temperature of 7.2 °C. If the GSM maximum (max) temperature, however, was >26.7 °C, an adjusted max temperature was calculated by first subtracting 26.7 °C from the GSM max temperature and then subtracting the GSM mean temperature. Then the growing degree days (GDDs) were summed by subtracting the base temperature of 7.2 °C from the average of the GSM minimum (min) and adjusted max temperatures. This method produced a cv of 3.96 compared to 4.13 for the standard method of summing over the entire growing season, the mean temperature minus the base temperature of 4.4 °C. The method with the lowest cv for predicting last harvest was to sum, over days from planting to harvest, the difference between the GSM max temperature and a base temperature of 7.2 °C. If the GSM max temperature, however, was >29.4 °C, the base temperature was subtracted from 29.4 °C and not the actual GSM max temperature. This method produced a cv of 3.71 compared to 4.10 for the standard method of summing over the growing season, the mean temperature minus the base of 4.4 °C.