I built a greenhouse for my seedlings with cardboard boxes I had around the house.

Step 1: Why?

I live in a cold place where the last frost is predicted to be happen around half May, so gardening season is really short and I wanted to give a good head start to my plants this time.
I don't have too much cash to spend on this project so after browsing the web for used greenhouse shelves with no success, I decided to build one by myself.

Step 2: Gather All the Materials

I have the habit of stashing cardboard boxes that I find somehow useful. I knew I would eventually need a ridiculously big box.

I also had some foamboard I collected from a Radio Shack that was closing near my house, this things are very useful to hold molds for small concrete projects.

You'll also need tape, packing tape or duck tape both work great. A hot glue gun, box cutter, scissors, a pencil and a ruler. Also clear plastic wrap, I used the plastic I just ripped off my windows from the winter season. It's the kind that gets tense with a heat gun.

Step 3: Reinforce the Box Corners

Since I had this box stashed for a while, it wasn't in its best condition, so I decided to reinforce all the corners and the frame of the squares I was going to cut out later.
I was feeling lazy to take measurements, so every strip is the width of two rulers.

I used hot glue to stick everything in the inside of the box.

Step 4: Add Some Tension

The box still felt a bit wobbly so I did this X with raffia I had in my toolbox. This made a huge difference, the box was feeling a lot safer to hold its shape after this step.

Step 5: Cut Off the Windows

Use the reinforced corners you just glued as a guide.

Step 6: Make a Roof

The main reason I needed a greenhouse for my plants is to protect them from my cats, one of them loves to "hunt" plants and will kill every green she finds. So I needed a roof that they won't try to climb.
I forgot to take more pics of this process but it's kinda simple.
I taped too foamboard together and made them into a triangle shape, once I got the height I wanted I just cut two triangles with that height and the length of the greenhouse.
After that I cut windows off them too.

Step 7: Add Plastic Wrap

Pretty much straight forward, tape the plastic to the windows, from the inside. I'm planning to stretch the plastic with the heat gun but it was a bit late for doing that noise, I'll do that other day.

Step 8: Place Your Plants Inside.

You are done, put the thing together and place your plants inside.

As you can see, there are already three bean plants, some marigolds, growing fast. The peppers and tomatoes are starting to sprout too. The other tray is my cats' wheatgrass, I keep it inside the greenhouse so it doesn't get cold during the night.

I'm thinking about adding some led strip grow lights this week, so I'll post an update after that. In this moment I use a single bulb grow light and my sad lamps, which seems to emit a similar light to the one plants need.

Hope you liked this guide, hope you can use this to build cozy homes for your seedlings.

Step 9: Updates

1. I added a humidity and temperature measuring thing I use in my bedroom to check humidity levels (heater dries the air inside the house very quickly). The regular temperature inside the greenhouse is around 22°C - 19°C.

2. I bought another grow light. I've been using a fcl bulb grow light but I'm not a fan of them because of the risk of them breaking and me having to clean up and ventilate the room to avoid breathing the toxins. I found a decent led lamp online, which arrived today. It's been on for about 10 hours, it's warm but not hot. I'll turn it off soon. The energy consumption is 12w, the previous lamp was 13w. Not much difference, I just prefer not to use fcl.

3. I'm using the cardboard I cut off for the windows to cover the greenhouse, during the day the light doesn't bother me that much but after the sunset it does produce a weird reflection around the living room. I may sew something with reflective material if I find something cheap at the dollar store.

Everything looks good so far. The small succulents my friend gave me before winter are enjoying the new environment. Three new plants started growing, I thought those seeds were goners, but it seems like they just needed a warmer environment.