It would be cheaper and easier for you to go to a local nursery and buy used black pots. You have to start a small plant in a small pot...seedlings in pots or divided trays 1"-3" diameter. When they get too big for the pot, roots coming out of the drainage holes and thick enough to block the holes, or you see more roots than soil when you slide the plant out of the pot...plant needs water all the time...re-pot the plant into a 6" to 1 gallon size. When it grows out of that, re-pot into a 2 or 3 gallon size...the proportion of roots to soil is important so that the plant is actively taking up water so that there is plenty of air being replaced in the soil.
A lot of people plant seeds in the size pot they think the plant will need when it is mature. Imagine a 5 gallon pot with a baby basil seed germinating. You might even know that soaking the soil is not a good idea and only water the seed/seedling a little bit each time. The water you dribble in each time, 2 or 3 or 4 or 5X per day is being pulled away from the plant by the dry soil and capillary movement of water through the soil's pore spaces. Might work but you can't miss a single watering or dribbling or you've lost your plant. Never plant a plant in a too-big pot.
Re-potting is a lot of work. Nurseries have to use pots to sell plants individually and it is a huge job. That is why big plants in large pots are expensive! There is a lot of work to get that plant to that size! It also means you should always check out the roots on plants you want to purchase! A lot of nurseries, in order to keep the costs down, miss a number of these re-potting steps.
I JUST built my greenhouse. I know I'm not going to be selling plants, how much work re-potting is and I don't want to have to purchase soil so I built my greenhouse over the best area for a garden. I've got all my purchased, potted plants I want to grow in my beds secure in a heated greenhouse (that's what most plants in the nursery were from early in the spring and it is up to the customer to' harden' these plants, get them used to full sun, not so great soil, wind, rain and temperature changes.) My seeds are started in grower's trays, re-potted into 2-3" pots, some of the plants I bought that were in 1 gallon pots are now in 2-3 gallon pots. I am trying to get my beds prepared and get my potted plants planted in the garden beds before I have to re-pot again. If my beds were already for plants, double dug to make raised fluffy beds, compost mixed in my very sandy low organic matter garden soil, trenches at the bottom of my beds so that water doesn't erode beds and walkways, raked smooth and firmed down...I could just plant seeds in my soil or my plants whatever size. But, sigh, I just learned that there is no 'last frost' here and I HAVE to have a greenhouse.
Now I understand you were building beds in 'pots.' What if you get a disease? You'll have to get rid of all the soil AND how would you sterilize your pots? The wood would probably have to be disposed of or you'd have to be careful when you rotate your plants each year. I'd either use your soil beneath your greenhouse (better harvests!) or get plastic pots and if they are used, wash well in bleach and water before putting potting soil in the pot. Leave at least an inch but not more than 2" for large pots so that you can water properly. Get used to the weight of a well-watered pot, makes it easy to tell if you need to water again.
And DO NOT Use rocks, gravel at the 'bottom' in hopes to improve drainage. This actually makes drainage worse as you've created a 'perched water table' allowing water to move out of the soil ONLY after the soil is completely saturated. It would be good to put pots on wire shelves or use tiles so that they are off the ground, solid surfaces to increase drainage.
My greenhouse needs opening up! I'll check back later...use those boxes upside down to lift plants up for better lighting...or air circulation!
