Does it have to have been planted? Forests grow naturally, not because people plant them - blacbird's posts about squirrels is just one example of how this happens.
When I was a kid, there was a hurricane on the South of England which felled huge numbers of trees in our woodlands, pretty much flattening entire historic forests. People tried to grow new trees but attempts to grow them back didn't work very well... meanwhile the forests left to their own devices grew back all by themselves and had gone back to being healthy, self-sustaining woodland, while the planted trees looked small and kind of sorry for themselves. Deforestation is a problem caused by people keeping on chopping down more and more trees and/or grazing animals on the land which used to be forest. When the people and their farm animals go away, the forest grows back pretty quickly, as long as there's still enough topsoil (if that goes away it can end up as desert, especially if the climate's hot and dry - in a temperate, wet climate desertification isn't really an issue).
Managed forests do exist (e.g. for sustainable sources of wood where they plant trees to replace the trees being cut down) but in most cases it works just as well or better to leave the land alone as it does to physically plant trees.
Get an A-level (UK school year 12 and 13, or equivalent leve in other countries) biology textbook and look up succession (ecological succession) - this is the process by which barren land gets colonised gradually by more and more life. It also refers to the process whereby land that isn't forest becomes forest (it's a later stage in the natural succession of land). And other examples of how ecosystems in any location change over time (changing from one ecosystem to another, etc.) Usually some kind of forest is the end result of succession (called a climax community), i.e. it all becomes stable and there are no more lifeforms (apart from industrialised* humans and their chainsaws selling to a global market that wants more and more and more and more and more) to move in, unless there are massive changes in environmental conditions.
*I'm not going to tar all humans with the same brush. There are plenty of humans that can live in forests without destroying them.
In some cases, the prevention of further succession is necessary to protect ecosystems, for example here in Dorset where I live there are areas of heathland. Traditionally, people grazed sheep and stuff on the heath and this prevented trees from growing, and the heathland ecosystem was stable and home to various heathland wildlife. In modern times, without grazing animals, heathland quickly gets taken over by trees and starts to become woodland. People remove young trees of certain species in order to protect the heathland and various wild species (such as the Dartford warbler) that rely on it. Oaks tend to be one of the later species that move into forests and oak woodland is pretty stable. Oak woodland isn't 100% oak though, there are all kinds of other trees, but oaks tend to dominate. A lot of British woodland is oak and it grew naturally.
If your scenario's on an alien plant where there was no life before humans came along, then I'd find it more plausible that they had some process whereby they could introduce hundreds of species in the right order for succession to happen (lichens first, then mosses, etc) than to just go there and plant a load of oak trees. You can't plant oaks in barren land and expect them to survive. Even planting trees in Surrey and other places in SE England that are famous for their woodlands and already have the right soil for trees to grow didn't work out after the hurricane. Squirrels and other mechanisms that nature has of growing life where life will grow worked way, way better.
Traditional ways of managing forest involves 1. the forest was there before the people, 2. cutting down the trees that you need (to build dwellings, etc) and 3. when the people move way the forest grows back. Traditional small scale farming communities don't cut down enough trees to endanger the forest. They don't need to do anything to make the forest grow back. If they're grazing animals then that will prevent forest growing back on the land that's being grazed, but when the animals are moved to other bits of land, the trees will grow back.