One problem with growing plants above the 45th parallel is the short growing season and the lower amounts of sun energy to warm up the plants and ground to help heat loving plants grow. There are a few solutions: global warming, outdoor heaters or a greenhouse. Not all are practical, cheap or desirable however a cheap sheet of clear plastic and some flexible sticks/pipes can provide the extra warmth your little plants need.
So how does a greenhouse work? First off, the sun emits a wide spectrum of radiation from deadly ultraviolet to the warm infra-red. Technically it emits the entire spectrum from gamma to LF radio but the larger portion of EM energy is in that range. The magnetic field and then the ozone actually absorb and/or divert the dangerous energies so that life can exist on this planet. So when the shorter wavelength (higher frequency) light enters the greenhouse it is absorbed and then radiated as a longer wavelength (lower frequency) infra-red radiation. The plastic and/or glass of the greenhouse walls will reflect this radiation keeping it inside the greenhouse and thus the greenhouse stays warm.
Since the effect of the greenhouse actually multiplies as the surface area increases you will want to group the heat loving plants together and then put a greenhouse over them together. To build a simple greenhouse you will need some type of support and a transparent covering. A cheap way is some PVC pipes (1/2 inch), clear thick plastic (8 mil) and some rebar. The length of the pipe should be enough to go over the garden bed and make a loop.
You will have the pipes cross over the planter area meeting in the middle (see the diagram), and hold the ends into the ground using the rebar, which the pipes just go over. To hold the PVC together in the middle take a 2" inch bolt with nut and drill a hole where the pipes meet to put the bolt through. Once the frame is set up put the plastic over the structure and fasten it to the poles. (Zip Ties work great). Please note that if you do not use UV resistant plastic it will degrade over time.
Remember that in these greenhouses it can get very hot and so it will need to be monitored and opened to now cook your plants before they reach your pot!