Sowing the Katsura Tree (Cercidiphyllum japonicum):
The seeds benefit significantly from a short period of cold, moist stratification before sowing. Pour the seeds into a small container with very slightly damp sand or vermiculite, mix well, then place the whole container in the refrigerator (3–5 °C) for one to four weeks. This "mini winter" awakens the seeds and improves the uniformity of germination.
After this cold period, sow from late winter to spring in a seed tray or shallow box, using a very light mix: specialist seed compost or fine, sieved compost, possibly mixed with a little perlite or sand. Katsura seeds are tiny and need light to germinate: scatter them on the surface of the substrate without covering them, or barely cover them with vermiculite. Water with a sprayer to avoid displacing them. Then place the tray in a bright location, out of direct scorching sunlight, at a gentle temperature of 15 to 20 °C. The substrate must remain evenly moist, never waterlogged. Germination usually occurs within two to six weeks, sometimes somewhat unevenly; it is important to keep the trays in place even if emergence seems delayed.
When the seedlings have two to three well-formed true leaves, they can be pricked out into individual pots. Use a slightly richer mix: good potting compost, supplemented with a little garden soil and sand or perlite. Then place the pots in bright partial shade, preferably outdoors as soon as temperatures allow. In the first year, growth is concentrated as much on the roots as on the top growth; it is best to keep the young katsura trees in pots for a whole season, or even until the second spring in cold regions.
Undertake the final planting in autumn (October-November) or early spring. The katsura tree likes deep, humus-bearing, damp to moist but well-drained soils, neutral to slightly acidic; it tolerates a little lime if not in excess. Avoid very dry, compacted or waterlogged soils in winter.
Prepare a wide, deep hole, loosen the soil well and mix in mature compost or leaf mould. Place the young tree at the same depth as it was in its pot, taking care not to bury the collar, backfill, firm in and water generously. A thick mulch (dead leaves, RCW, coarse compost) will help keep the soil moist and suppress weeds. Choose a position in non-scorching sun or partial shade, in a spot sheltered from cold, drying winds to protect the young spring foliage.
For the first two or three years, monitor watering carefully in summer: the soil should never dry out completely at depth.
