Even from Central Europe you can get to Portugal without leaving a carbon footprint by flying. Euro Lines is the biggest Inter-European bus company. But don´t get up your hopes – Those long distance busses are not cheaper than a flight. And those 36 hours on the bus from Germany are virtually a pain in the ass.
From Spain or France it’s a completely different story. From Barcelona hopping on a plane is a lot more comfortable, but from Madrid and southern Spain, Sevilla, Cádiz, and Tarifa, the busses only take a couple of hours and should be cheaper than even the cheapest budget flight. ALSA and Euro Lines are the bus companies you are looking for. They also have busses at night time that can save you a night in a hotel. And they will get you to the ferry to Morocco if you want to follow the endless summer after a Portuguese Autumn.
From southern France you just need to catch a bus to Iron. The trains to Lisbon go at night and will only set you back around €60. Tickets are available on the Portuguese railway company’s website cp.pt.
All international busses and trains should be booked online.