A while ago i saw a video on Youtube where someone used an old aquarium as a mini greenhouse, which gave great results. I have started some tomato and other plants indoors and my windowsill isn't in the direct sun all day, so now that it is getting warmer here i made a little cold frame out of a plastic box (i don't have a usable aquarium) to test if this would work as well. The whole box including the lid is made out of transparent plastic, so the sun gets through and it warmed up really well. I don't know anything about using greenhouses, but does anyone here know if it would be a good idea to cover some plants in my garden with plastic boxes when the growing season has started to give them extra heat and humidity? The inside of my box quickly got covered with condense, so i opened the lid a little, but if i use them in the garden i won't be able to open anything on the top because the boxes will be upside down. Does anyone know if the condense will block too much of the direct sunlight or otherwise harm the plants (diseases, fungus,...).
The big worry with condensation is diseases. Especially with small plants, too much humidity is fatal. So you must figure out a way to ventilate. I don't know how much money you can afford to spend, but floating row covers would do what you need them to do. Check out, for instance, Peaceful Valley's website.
If you want to experiment with the plastic boxes, you could -
drill ventilation holes in them prop them up a few inches from the soil during the day use boxes that are at least twice as tall as your seedlings, use them right side up (cut the bottoms to accommodate your plants) and take the lids off during the day (if it's not raining) and replace the cover at night
Those teepee-shaped tomato cages that you buy at big box stores? They are worthless for supporting full-grown tomatoes, but they can make great plant protectors for not huge amounts of money. Use some bent wire to staple them to the ground, wide end down, narrow end up. Wrap them in clear plastic - heavy weight painter's plastic works - and fasten with duct tape or clothespins (you'll have to pleat the plastic to get a reasonable fit). Leave the top open. Your plants will be protected from wind, you will get a noticeable amount of heat gain, and the open top will prevent excessive humidity. By the time the plant starts to outgrow it's teepee, it should be warm enough to remove the teepee completely. For plants that will need support, you can then surround them with a proper cage. I make my cages from woven fencing, from four panels at least four feet tall by 18" wide, fastening the panels together with hog rings. They easily fold flat for next year.