Testing A Seed Starting Setup

For Christmas this year, I got a grow light setup. I'm planning on putting a timer on the light and planting some seeds next month before I go away on a 3 week trip. Hopefully they'll be sprouted and happily growing by the time I get back. The problem is how to make sure they stay watered while I'm gone. I'll be growing the stuff in seed trays which will be sitting inside a bigger tray (a flat). Underneath the flat I'll have a heating pad that should help keep it a bit warmer than it would otherwise be. It will look something like this:

There are a few things to note in the picture. First is that some of the trays look like they have dirt in them. They do. I planted some arugula because it is very fast growing and might actually be large enough to produce a bit of useful greens before I leave. I also planted a few onions just to try growing onions from seed rather from onion sets or transplants. Onions take a long time to grow from seed, so you need to start them in mid-winter if you want to grow them from seeds to onion bulbs. Another thing to note in the picture is that I put a bunch of empty trays in the flat. I did that to reduce the amount of evaporation and also heat loss. I also put some rags under the flat to insulate it and help keep the temperature up. The heating mat says it should raise the temperature 10-20 degrees above ambient depending on whether you insulate it or not. Since its in my somewhat chilly basement, I'd like it to be as warm as possible. Lastly, note the pop bottle in the picture. I said "pop" because I fall within the blue area on this map. The pop bottle is my attempt at a simple automated watering system.

I considered a couple more complicated watering system ideas that would probably work, but in the interest of saving my free time and money I decided to try the simplest idea and hope it works. If you take a full pop bottle (or any relatively stiff and airtight container), fill it up with water, flip it upside down, and quickly put the open end in water, you'll notice that the water doesn't all flow out. It wants to, but a vacuum forms in the bottle, and the keeps the water from flowing out. If you then raise the opening just above the water, some air will get it, which lets water out (equal to the amount of air that goes in). I'm pretty sure that is how the reservoir on our cat fountain works. When the water level gets low enough to let in a little air, the water comes out and raises the water level until the it is high enough to not let any air in anymore. I'm going for the same idea with my system. As you can see in the picture below, I modified the bottle a little bit.

I cut out a piece of plastic, drilled a hole slightly smaller than the opening in the bottle, and cut a channel from the edge of the plastic to the hole. I then caulked the plastic to the opening of the bottle. The plastic does two things. First, it stabilizes the bottle and helps to keep it from falling over. Second, it raises up the the opening of the bottle slightly. I tried to set it such that the bottle would be a height where it would keep the water level just high enough so that comes into the very bottom of the seed trays (they have little holes in the bottom of each). I'm thinking that the water should wick up into the soil and keep it wet. Its possible that this will either keep it way too way or not wet enough. That is why I am testing it out now. I'm going to try to not touch it for a while and see how well it works. Check back for an update!