Degree-days to harvest

Single sine with horizontal cutoff

DD = (1/π)·[(c − LT)(θ₂ − θ₁) + a(cos θ₁ − cos θ₂) + (UT − LT)(π/2 − θ₂)],
a = (T_max − T_min)/2,  c = (T_max + T_min)/2,
θ₁ = arcsin((LT − c)/a),  θ₂ = arcsin((UT − c)/a)

The general expression for a day crossing both thresholds. A day entirely above the upper threshold yields DD = UT − LT: the upper threshold means the development rate stops increasing above it. A day entirely between the thresholds yields DD = c − LT.

Corn GDD method (threshold substitution)

DD = (min(max(T_min, LT), UT) + min(max(T_max, LT), UT))/2 − LT

Used for sweet corn. Both daily extremes are brought to the thresholds before averaging; with T_min = 40 °F, T_max = 90 °F, LT = 50 °F and UT = 86 °F the result is 18 °F·day, whereas plain averaging gives 15 °F·day.

Harvest date

the first day for which S = Σ DD_i ≥ S_h

Summation starts at the planting date. Past days come from the archive, subsequent ones from the mean daily contribution over whole years of the archive. The lower and upper bounds of S_h across the cultivars of the source give an interval of dates.

Model parameters

Tomato: thresholds 45 and 92 °F, 1844–2010 °F·day across four cultivars (Table 6). Sweet pepper: 52 and 100 °F, 1211–1447 °F·day to first green harvest and 1682–1998 °F·day to first ripe harvest, four cultivars (Table 5). Cucumber: 50 and 90 °F, 946–1211 °F·day direct seeded (six cultivars) and 805 °F·day transplanted (one cultivar, Table 2). Sweet corn: 44 and 86 °F, 1441–1883 °F·day direct seeded and 1934–2014 °F·day transplanted, corn GDD method (Table 4).

Applicability

The models were built from field trials of 2013–2015 in the Willamette Valley, Oregon, on bare ground without plastic mulch. The source states that degree-days to maturity can still vary in different climates and recommends calibration against the grower's own records.

Relation to accumulated temperature

The two are not comparable. Accumulated temperature is Σ T̄ over days with T̄ ≥ T_b, counted from January 1; here the excess over the lower threshold is summed from the planting date. Conversion: 1 °F·day = 5/9 °C·day.