Hi Marilyn,

There are a few factors that determine when it's okay to start planting seeds or transplants.  The USDA plant hardiness zone map provides information about how cold the winter gets and average annual freeze dates.  Yankton is in zone 5a, so the last frost is typically between April 15-April 30.  However, every year is different, so it's important to look at the weather forecast to make sure temperatures aren't going to dip below freezing in the next couple weeks. Checking for freezing temperatures is important for sensitive transplants.

For planting seeds, different vegetables have different germination temperatures.  Many cool season crops can be planted at cool soil temperatures.  I have attached a table that lists the soil temperature for seeding and transplants.  This table was taken from the Extension publication Vegetable Gardening in South Dakota.  I've also attached the full publication.  

The following Extension publication lists the minimum and optimum soil temperatures for germination of many vegetables.  It's best to check your soil temperature before planting seeds of the warm season vegetables.  

https://extension.oregonstate.edu/gardening/soil-compost/soil-temperature-conditions-vegetable-seed-germination

Please let me know if you have any questions about specific vegetables.  Happy Gardening!

Robin