We have investigated the kinetics of the uptake and the translocation of nanoparticles in plants. Nearly monodisperse NaYF4:Yb,Er nanocrystals were used, either spherical particles with a diameter of 14 nm or nanorods with a length of 41 nm and a diameter of 22 nm. After watering pumpkin seedlings (Cucurbita maxima) with aqueous colloidal solutions of the particles, intact nanocrystals in the plants were detected in vivo by exciting their upconversion luminescence with a near infrared laser diode (978 nm). The particle concentration in different plant parts was determined by X-ray fluorescence spectroscopy (XRF), making use of the high sensitivity of Yttrium in XRF measurements and its low natural abundance in plants. The results show a fast uptake and translocation of the nanoparticles in all plant organs within three hours. The smaller particles move in the plants faster than the larger ones.