Transplanting Plumeria seedlings should be done at the right time to ensure their successful establishment in their new location. Here are some guidelines on when to transplant Plumeria seedlings:

1. Wait for Adequate Growth: Plumeria seedlings should have reached an appropriate size before transplanting. They should ideally be at least 6-8 inches (15-20 cm) tall with multiple sets of true leaves. This size indicates that they are strong enough to handle the stress of transplanting.
2. Consider the Season: The best time to transplant Plumeria seedlings is during the growing season, which typically aligns with late spring through early summer. This period provides the seedlings with the warmth and longer daylight hours they need to recover and establish themselves in their new location.
3. Avoid Extreme Heat: While transplanting during the warmer months is ideal, avoid transplanting on extremely hot days. Choose a mild, overcast day or early morning when temperatures are cooler to reduce stress on the seedlings.
4. Prepare the New Location: Ensure that the new planting location is well-prepared. The soil should be well-draining and amended with organic matter if necessary. Whether you are transplanting into the ground or into larger pots, have everything ready before you start.
5. Handle with Care: When transplanting, handle the seedlings with care to avoid damaging their roots or stems. Gently lift them from their current container or location, taking as much of the root ball as possible.
6. Watering: Water the seedlings thoroughly after transplanting to help settle the soil and reduce transplant shock. Be sure to keep the soil consistently moist but not waterlogged in the days following transplanting.
7. Protect from Wind and Direct Sun: Provide some protection from strong winds and direct sunlight for the first few days after transplanting. This can help reduce stress and allow the seedlings to acclimate to their new environment.
8. Monitor and Care: Keep a close eye on the transplanted seedlings in the days and weeks following the transplant. Ensure they are adjusting well and continue to provide proper care, including appropriate watering and fertilization.
9. Mulch: Applying a layer of mulch around the base of the seedlings can help retain soil moisture and regulate soil temperature, further reducing stress on the plants.
10. Gradual Acclimatization: If you are moving the seedlings from an indoor environment to an outdoor one, gradually acclimatize them by exposing them to outdoor conditions for increasing durations over the course of a week or two.

Transplanting Plumeria seedlings requires careful timing and handling to minimize stress and ensure their successful growth in their new location. By following these guidelines, you can help your seedlings establish themselves and thrive in their new home.