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As long as the plant has been
As long as the plant has been properly cared for -- given enough light (about 6 to 8 hours at least), good potting mix with soil nutrition, proper watering, no pests/diseases, etc. -- then it can be as healthy as one planted directly outdoors. There’s a chance that it may be even healthier, as there is more opportunity for animals, cultural, other problems to pop up and disturb seedlings sown outdoors. When transplanting, choose a good site and be careful with the roots—try to disturb them as little as possible. As for watering, sunflowers need about 1 inch of water per week. They can tolerate drought for a time. The soil should not be soggy. If the soil feels dry (say, down to about 1 inch deep from the surface), then it is time to water.