Budburst date of Quercus petraea is delayed in mixed stands with Pinus sylvestris

•Budburst of sessile oak and scots pine was monitored for seven years in 18 plots.•Budburst dates in monospecific stands were compared with dates in oak-pine mixtures.•There was no stand composition effect on scots pine budburst date.•Sessile oak budburst date in mixed stands was delayed by 2.2 days on average.•The delay was more pronounced in years with early budburst.