I live in zone 5 - central Missouri. We have had a cold spring and then suddenly it was 80 degrees. The tomato plants I just put out in the garden now have thin yellow leaves (like a sunburn). Why does this happen? Should I have hardened them off better before setting them out? Will this hurt them or will they grow and just continue putting on new leaves?

Thanks!

Jeanna

Yes tomato plants can sunburn. Moving the plants quickly from inside to full sun outside can damage the leaves. As to why this happens, lets examine things from the plants point of view. It begins growing in a low light area, so it adapts it's leaves in order to collect all possible light. It is unlikely to experience a sudden change in light because the plant can't move itself. Suddenly the plant finds itself dealing with full sun and it leaves can't shift gears fast enough to avoid damage.

I think more of white or gray spots on the leaves when I think of sunburn, but I may not be familiar with all the symptoms. Yellowing of leaves could also indicate a nutrient deficiency. This may be because the plant hasn't had enough time to develop it's roots yet.

I believe your plants will probably recover, it depends on the extent of the damage. How do the younger leaves look? If they are still darker green, they should be ok. Most likely you will have probably just checked the plants growth by a few weeks.

I'm still working out how much direct sun plants should receive each day as it transitions, but perhaps another gardener on the forum can explain that a little further.

Well, down here, if you plant tomatoes, you will get yellow leaves no matter what you do, just because of the humidity. Does not matter how you water, mulch, no mulch, which ever variety you plant, you will get yellow leaves, starting at the bottom of the tomato plant.
Does not seem to bother fruit production.

Yellowing plant leaves, depending on how they yellow, can indicate a nutrient deficiency starting with Nitrogen but including Potassium, Magnesium, Zinc, Iron, Sulfur, Boron, Copper, Manganese, and Molybdenum among others. This could be the result of a soil nutrient deficiency or simply because the soil is too cold for the plants to uptake those essential nutrients.
If this is caused by a too cold soil the plants will self correct things as the soil warms up, but if it is because of a soil nutrient deficiency you need to determine that and take steps to correct this problem.
Any time you move plants from a very sheltered environment to that very harsh outside they need to be "hardened off", prepared slowly for life outside, and that should take a week.