The method Ribes uva-crispa is based on leaf-out of gooseberry (Janssen 2009).
It was developed by the Germany's National Meteorological Service
(Deutscher Wetterdienst, DWD) and is more robust against early starts than
common simple meteorological procedures.
Calculation
starting from 18th February sum all day degrees above 0 °C (daily average air
temperature). Vegetetaion period starts at the day when 164 is crossed.
# start day: February 18th == 49 doy
# only Tavg over 0°C are summed
df[df$DOY < 49 | df$Tavg < 0, 'Tavg'] <- 0
# find day where cum day degrees cross 164
start <- tapply(df$Tavg, df$year, FUN=function(x){
x <- cumsum(x)
min(which(x > 164))
})
References
Janssen, W. (2009)
Definition des Vegetationsanfanges.
Internal Report, Deutscher Wetterdienst, Abteilung Agrarmeteorologie.