As a result of recent flooding, many questions have been raised about the safety of consuming produce from gardens that were under water for a day or two. How concerned do you have to be about using garden produce after a flood depends, to a large degree, on whether that flood water was "clean" or contaminated with sewage or industrial pollutants. Raw sewage contains bacteria that can cause illness if contaminated fruit or vegetables are eaten.
The most conservative answer — one that eliminates any and all risks — is that you discard all produce that was covered by contaminated flood water. Another choice would be to use root crops such as carrots, potatoes, and parsnips, but scrub them thoroughly, and cook them. Boiling kills any harmful bacteria or other microbes present. Leafy vegetables such as lettuce, cabbage, or spinach should be discarded because it's not possible to scrub them, and they have many ridges and crevices that could contain contaminated silt or bacteria.
Vegetables that result from flowers produced on growth that develops after flood waters subside should be OK. To increase safety, cook them thoroughly, too, or at least wash them well and peel them, if possible, before eating. This could include tomatoes, peppers, eggplants, sweet corn, squash, cukes, and similar vegetables.
Examine produce carefully before picking it. If it is soft or cracked, or has open fissures where contamination might have entered, throw it out. Produce from plants that survive flooding with water that was not contaminated should also be discarded if they are bruised, cracked, or otherwise blemished. Root crops from "clean" flooding should be OK if the upper parts of the plants survive essentially undamaged.
Do not use produce from plants that yellow and wilt, regardless of the type of flooding... It's not worth the possible risk.
When planting gardens that have been covered with considerable floodwater it is best to take a soil test, as the nutrient content of the soil may have been changed by the flood waters. Soils that have been covered with floodwaters should be tilled at least 6 inches after they have dried out before planting a new crop