Care Before Planting
For field planting, wait until the soil warms up. If planted too early, the wet cold soils will cause the cuttings to rot before they have a chance to sprout. In other words, it is a race between sprouting and rotting. Put the odds in favor of sprouting by waiting until the soils stay warm. In East Tennessee, that would be the last week in April or the first week in May, or when the ground temperature stays above 55º F.
Upon receipt, refrigerate until you are ready to field plant. They are packaged so that they can be stored for about six weeks in refrigeration. You need to check weekly for condensation on the plastic bags. If it develops, simply remove the roots from the bag, let them dry out for a day, and replace into a new plastic bag. Wrap in a dry paper towel. The paper towel should wick any surface moisture off the root cuttings.
Another method is to place into a soil-less potting medium, such as sand/vermiculite mixture, and simply leave outside in the weather until you are ready to field plant. You can wait until you see the first signs of sprouting as indication for planting.
If the root cuttings are small, it might be best to pot the cutting into ½- to 1-gallon containers and let them grow in these pots for the first few months. You can expect a higher mortality (up to 75 percent) on cuttings less than .25 inch in caliper.
Although not absolutely necessary, it is a good idea to soak the root cuttings in a fungicide for at least an hour before field planting. My current choice is Infuse made by Bonide and purchased at my local farm co-op. It is broad spectrum and systemic, and can last up to four weeks. It is good insurance to help prevent field rot.
Loosen the soil before planting by digging a hole, then refilling the hole with the loose soil dug. I prefer using a tractor with a chisel plow to break and loosen the soil. Weeds must be kept under control the first year, so I use a pre-emergent product call Princep 4L mix with Roundup after tilling the soil. It works for about six weeks. After that I hole/mow.
Planting orientation is also important. The cuttings need to have the same vertical orientation to the ground surface as when dug. The cutting will always sprout on the end that was originally closest to the ground surface. If planted upside down, it will sprout toward the center of the earth! I prefer to simply place the root horizontal and let it chose the sprouting end.
Planting depth is also important. Place the sprouting end of the root cutting just below the ground surface to no more than a half-inch deep.
Mark your planting with a survey flag to add in weed control and watering, both of which are very important throughout the summer months.
After Care Planting
Do not water your root cuttings for the first few weeks. There should be enough moisture in the spring soil to keep the cuttings healthy. Too much water promotes rot. You can water once the sprout has emerged and the weather turns dry.
Keep the weeds under control the entire summer. Use whatever means it takes. Paulownia is very intolerant to any type of shading or competition.
Allow the cutting to grow naturally. What you need to do during the first year is just let the root system develop. Plan on pruning the trees the beginning of the second year to develop the first log of your future Paulownia lumber.
In China, root cuttings are the method of choice for Paulownia propagation. It is their method of choice for cloning. It is a low-tech, highly successful technique that has been proven to work well for centuries. If you have not ever used root cuttings, you might give them a try.