On a continental scale, Europe is small. But it is densely populated and its major gardens are far apart in terms of travel time. One could gain a wide appreciation of South European garden history in the course of three week-long trips. The first should be to Egypt - not part of Europe but a western country which had a profound influence on Europe's gardens. The tour should be done by plane-and-train/boat. The other two tours can be done by rail-and-cycle or fly-and-drive. They allow one to concentrate on particular design styles:
Egyptian gardens: Ancient gardens.
Italian gardens: Roman Gardens; Early Renaissance, High Renaissance, Mannerist, Early Baroque
Spanish gardens: Late Medieval; Twentieth Century
Europe - Image from Garden History
Italian Garden