Answer You are going to have to be prepared to pot them up often if you want them to be a blooming shrub. They can get as big as a tree when they are planted in the ground, but because their roots are restricted in the pot, they won't get as big. From personal experience, I had three that I had potted up from starts taken from the base of an old tree, and I moved them to my new location. They sat in those same pots for over ten years. They never grew and never bloomed. I just planted them in the ground this spring and they have doubled in size already. I didn't give them any special care. I didn't water them except when they got really dry, and they were in part sun and left out on a porch all winter. But the key to having a healthy blooming shrub is to make sure it has room to grow in it's container. Wait until the roots are filling the container. Pull the plant out and check it every so often, then plant it in the next size larger container. Don't try to skip this process, no matter how tempting it may seem. Water them heavily after repotting then, let soil dry before watering, then soak the soil heavily. In the smaller pot, the water is directed to the root ball, in a larger pot the water is pulled away from the root ball to the outer edge of the pot. To get it to bloom, it will need plenty of sun in the Spring. Put it in the sunniest, brightest spot you have in the early Spring. When it is through blooming, and the temperatures are getting hot, move to a place that has part sun, like in the morning only and shade in the afternoon. This is not a indoor plant so it will not grow in low light areas. You will want to prune off the dead flowers and prune it to shape it after it is through blooming. Discourage any new growth, known as sucker growth, at the base of the plant. Fertilize only if the plant looks like it is not getting enough nutrients. Use a flower fertilizer to encourage blooming. You can add a small teaspoon of time release fertilizer in the potting soil when you pot it up. There are other flowering shrubs that are better suited for containers then a lilac. But with some effort it is possible to have those beautiful, scented flowers to enjoy.
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