Answer: The best time to try to move it is in early spring as soon as the soil can be worked (not frozen and not too wet), or in the fall once it has dropped its leaves. Dig it with as many of the roots intact as possible (the roots should be growing deep and wider than when it was planted) and replant immediately at the same depth as it was growing. Water it well to settle the soil and mulch with several inches of organic mulch in a flat layer over the root area. Be prepared as the rootball will be large and very heavy. After planting, keep the soil evenly moist (not sopping wet) for the rest of the season until the ground freezes.
If you are transplanting it that is also a good time to prune it back. If you lost many roots at digging then you will want to cut it back proportionately. If you have a large overgrown plant then you may also want to prune it back to shape it better.
Here are the basic pruning directions for lilacs -- both routine and renewal. Since you are moving it now, you could go ahead and it cut it all back very short to the ground when you dig it. This will sacrifice this season's blooms but encourage new growth which you can then train. (It will also make it much easier to move.) Then next year you would start with routine pruning.
There are two ways to bring back a big overgrown lilac. The quick and dirty method is drastic but it works. You simply cut it off at the ground in the very early spring using heavy loppers or a chain saw. It is important to do it in late winter to early spring in order to allow the plant the maximum time to recover during the coming growing season. This prevents bloom for a year, but it is simple.
Or, you can prune selectively over a three year period. If you opt for this method, prune right after it blooms. In year one, take out one third of the trunks or main stems by cutting them off at the ground level. In year two, remove half of the remaining original old stems again cutting as close to the ground as you can. In year three, remove the rest of the orginal old stems. Meanwhile, each year, also remove any new shoots that are smaller than a pencil in diameter. After three years, you will have renewed the entire plant.
Maintenance pruning is done every spring right after the lilac blooms and consists of removing a few of the oldest stems by cutting them off at the ground, and also removing any shoots coming from the roots that are smaller than a pencil in diameter. This encourages an ongoing supply of vigorous new growth.
Lilacs bloom on old wood, meaning wood that grew the previous season. That is why pruning is usually done right after the bloom season and not at other times of year.
Good luck with your lilac!
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