For Sweet Oranges, it can be difficult to predict the timing of flowering, before harvest. For this reason, researchers have created the Citrus Flowering Growing Degree Day Model, which uses heat measurement and time to predict flowering timing of Washington Navel Orange in Southern California.

Setting the Biofix Date

For this model, the biofix date, or date from which to begin accumulating growing degree days, should be set to January 29 for Southern California.

Calculating Growing Degree Days

For this model, a lower temperature threshold of 49F is used to calculate growing degree day accumulation beginning at the time of planting. Growing degree days can be calculated from daily high and low temperatures, or via the Pest Prophet app, which uses hourly temperature data for a given field. This can be used to flowering  based on growing degree day accumulation which will occur at 10% of anthesis, approximately at 536 GDD (F) after the biofix date. 75% of petals will have fallen by 766 GDD (F).

Source:

Lovatt C.J., Streeter S.M., Minter T.C., O’connell N.V., Flaherty D.L., Freeman M.W., Goodell P.B. (1984) Phenology of flowering in Citrus sinensis [L.] Osbeck, cv. Washington Naval orange. Proceedings of the International Society of Citriculture 1, 186-190.

Bellows, T. S., Jr., J. G. Morse and C. J. Lovatt. 1989. Modelling flower development in citrus. IN Wright, C. (Ed.) Manipulation of Fruiting. Butterworths, pp. 115-129 (Chapter 9).