I had this problem last year in central PA. We had frosts on May
20th and 21st !!

I live in central PA too, and we had a low of 28 on May 21, but lows
of 34 on May 19 and 20.

USUALLY, I start my tomato seeds in March, and put the seedlings in
the garden around April 27, the AVERAGE date of our last frost.

I plant them deep, so only the top leaves area above ground. I put a
gallon jug of water right next to the plant, to absorb warmth from the
sun all day. At sundown, uness the forecast low is above 50, I put a
six-gallon plastic bucket over both plant and jug.

That whole gallon of water would have to freeze before the temp inside
the bucket woud drop to 32. In fact, water has a high specific heat
and it keeps the temp inside the bucket well above the outside
temperature.

By the time the plants are too big to fit under the bucket, I no
longer need to cover them.

I have never lost a plant.

BUT THIS YEAR, I planted my tomatoes last Sunday, a month later than
usual. It's been a terribly cold spring. Many nights down in the 40s,
even last week (41 on May 22); and quite a few days barely reaching
the 50s. It has rained 23 of the 27 days in May.

vince norris