Answer: Oscillating temperatures and periods of warm vs cool, frosts and no frosts are typical in spring in your region. You can routinely cover the beds at night with a spun fiber frost blanket to provide some protection once the seeds have germinated. Or, you can toss a fluffly layer of clean straw over the plants at night, and rake it aside each morning. When the weather settles, use the straw as mulch or add it to your compost pile.
Your seeds should be fine so far. The beets, turnips and Swiss chard are more cold tolerant than the lettuce, but the lettuce will handle frosts as well. If by chance it suffers freeze damage, you can replant; lettuce grows very fast. (You should be planting leaf lettuce at regular intervals anyway if you want continuous production.) To put it into perspective, some people overwinter the Swiss chard (and spinach) from a fall planting, and the cool-loving beets and turnips must be planted very early -- turnips hail from Siberia after all; lettuce sometimes self seeds from year to year, too.
There is no sure fire way to judge impending frosts. You have to watch the late night weather and see if they give a frost warning or the colder hard freeze warning, then make your own judgement. Or, routinely cover the plants every night. Put the cover on in late afternoon so it traps heat from the ground. If the day is extra cold, leave it on.
Microclimate does have a lot to do with it. You can begin observing and making notes as to where the colest parts of your yard and garden are, or you may notice it seems fairly consistent across the board. Also note if you have frost or don't have frost when it is predicted. You can even use a min-max thermometer at night to check how cold it actually gets overnight. This will help you begin to figure out your microclimate.
Gardening is imprecise at best, and we can't control the weather. The frost dates are averages, so typically the last one is either earlier or later. We just have to learn to live with it. It will be okay!
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