Question
I have a maple tree outside my home which gives off the twirling seeds every year and I've planted some seedlings in pots when they were about six inches tall. One is growing rapidly this summer and has grown 3 feet so far. I want to keep the tree in pots until I have a place to plant it, so how long can I keep a maple tree in a pot and how do you know what size pot to plant a tree in,(do you go by the height) and should you take them in for the winter or leave outside? Also, should you fertilize such a young maple and leave in full sun? (sorry for all the questions.)
Thank you!

Answer
It sounds like you are doing very well to have one that is 3 feet tall already. As long as you have a pot big enough for the roots to grow in, you can keep it in a pot. Example: Some of the trees you see in the nurseries are in huge containers.
Check the roots. Carefully take it out of the pot, and look to see if the roots have grown down to the bottom of the pot. Always pot up to the next size larger pot. Use good potting mix with good drainage. Press the soil around the roots, for good contact, and water well to get the air bubbles out. Keep the soil moist, not wet, for about two weeks, until it is growing again. Then let the soil dry more between watering.
Remember, the larger it gets, the heavier it gets, and the harder to handle. Also the larger the pot, the bigger the hole you will have to dig when you plant it. (I have six little trees in a six pack, that I plan to plant when they become one gallon size.)
It should do fine outside for the winter, unless you live in an area where the soil will freeze solid, and it won't be able to take up water. Put it in a protected place out of winds, and frost, like on a porch, or deck next to the house. Don't fertilize. It will do better in part shade, or filtered sun. Too much hot sun will burn the tender leaves.
The biggest problem in winter is temperatures changing rapidly from hot to cold to hot. If you see this pattern in the forecast. bring your tree in to a garage or some where that does not freeze until the temperatures stablize again. When planting your tree, this is important to consider as well. If you plant it where the sun shines on it during the day, and heats it up, and then it freezes at night, the tree will swell, and split the bark killing the tree. There is also the danger of heaving out of the soil, as it freezes and thaws. These trees are also susceptible to sun burn as mentioned before.
So choose your planting spot carefully.