@Blew Hiller has a good strategy to deal with this. While the plant is small, I would water out to a circumference of about 3x the plant size, trying to tease out the roots to grow laterally, and I would not try to saturate the container at first. Once the plant is established in the container and has a couple of sets of leaves, you can be sure that there are now roots all the way to the bottom of that large container. At that time, you are right... it is time to properly water the entire container but it is also very important to let the container dry all the way to the bottom between these complete waterings. The little plant just can't do this quickly, so while you wait for the bottom to dry out, water the top every 2 or 3 days with about 1/3 the amount of water it took to saturate the container. This will keep the top spreader roots active, and a little bit will go down the middle and add to the water table, but you don't want to water with enough that it adds to the problem down below. After 2 or 3 times of partial watering, the bottom will catch up, and then and only then, water then entire container again. As the roots get stronger, the time difference in this 2 stage watering will start to sync up and eventually you will find that the entire container is drying out in 2 - 3 days, and you can start watering properly from that time forward.
