The evaluation of potential biostimulants to be used in sustainable horticulture production is a crucial goal of research. Most research has focused on the effects of biostimulants on plant growth, and less on the effects on seed germination and seedling growth. This study evaluated the biostimulatory effects of mugwort extract on seed germination and seedling growth in several vegetable crops (onion, carrot, tomato, rapeseed, cauliflower and lettuce), in order to test its application as a potential biostimulant. The phenolic acid composition of the extract and the acids’ rankings were: homovanillic > gentisic > gallic > caffeic = chlorogenic > salicylic = syringic > p/m-coumaric = ferulic = synaptic = p-hydroxybenzoic. The extract of mugwort (at 0.2, 0.4, 0.8, 1.56, 3.13, 6.25 and 12.5 % w/v concentrations) was analyzed using Petri dish bioassays, quantifying its stimulatory effects on seed germination and the radicle and hypocotyl length of the seedlings, according to hormetic log-logistic models. The mugwort extract was not able to biostimulate all the tested species. Seed germination was stimulated in carrot (+70%) and rapeseed (+11%), while in the other species, no effects (i.e., onion, tomato and lettuce) or inhibition (i.e., cauliflower) were observed. Hypocotyl length stimulation was observed in all the species except carrot and onion, while radicle length was mainly inhibited by mugwort extract, except in rapeseed (+30%). The biostimulation effects of mugwort extract seem to be “specie specific” and “part of plant specific”, and need to be further investigated in terms of the involved substances and physiological aspects, although phytohormone activity is certainly involved.