Use it now. It is not a fertilizer. It is designed to reduce transplant shock. Do not apply to the foliage. It is to be used on the root system only. Read the instructions. Use as directed. Not all products are used the same way. In the future, look for the products that contain the B1. It is very important in preventing shock. Sandy

In my enthusiasum for the root stimulator, I negelected to comment on the most important part of your problem. Your plant is going to die. There is no real way for a mature tree fern to survive when it's root system has been damaged to the extent you suggest. The quantity of roots left cannot be expected to supply the amount of nourishment and water the foliage requires to survive let alone flourish. You don't say how the damage occured but obviously there was no preplanning involved. If the tree fern was being removed from a pot, it should have been carefully separated from the pot by sliding a large knife around the inside of the pot and the fern and soil ball wrapped carefully in burlap for transport. If it was being taken from the ground, there should have been a sharp edged shovel used to sever the roots between 6 inches and a foot from the trunk of the fern about 6 weeks before digging it up. (The larger the plant the larger the rootball.)This encourages new root growth close to the trunk. Using the root stimulator at this time is a good pretreatment and gives a better chance of survival. Lift the root ball carefully and lay it on burlap. Wrap it and the trunk firmly for transport. The less damage to the root ball the better.