In this study, the effects of long-term nitrogen deﬁciency (N 0.1 mM for 4 months) on growth, phenolic content and activity of phenylalanine ammonia lyase (PAL; EC 184.108.40.206) were investigated in the leaves, inﬂorescences and roots of yarrow (Achillea collina Becker ex Rchb.) grown in hydroponics. The antioxi-dant capacity of methanol extracts was also evaluated. Nitrogen starvation decreased plant growth and the leaves’ total nitrogen, amino acids, proteins, chlorophylls and carotenoids contents indicating that the primary metabolism was severely limited by low nitrogen availability. The amount of total phen-olics and the antioxidant capacity were higher in leaves and roots of nitrogen-starved compared to con-trol plants. The most abundant phenolic acids identiﬁed in yarrow were 5-O-caffeoylquinic acid (chlorogenic acid), and 3,5 and 4,5-di-O-caffeoylquinic acids. Nitrogen starvation signiﬁcantly increased the contents of all these compounds and the PAL activity in leaf and root tissues. Results suggest that hydroponics, with proper manipulation of nutritional factors, might be a suitable system for the produc-tion of compounds with health beneﬁts, such as caffeic acid derivatives, in yarrow.