We are getting a freeze warning tonight into tomorrow after 15 days of record high heat here in SW Ohio (and pretty much everywhere east of the Mississippi). All that record setting heat fooled a lot of fruit crops into thinking it was mid spring and time to flower and so they did. the heat also fooled a lot of gardeners into thinking it was mid to late spring and high time to plant crops out side.
That Ma Nature, wotta prankster she is. And boy, did she punk a lot of plants and people with her unseasonable (and if you ask me unreasonable) warm weather through most of the month of March.
So what to do if you have lots of fruit trees in full bloom? You can probably do nothing for most trees but you may not have to after looking at this chart. This shows that fruit trees can take quite a bit of cold before you start to lose most/all of the crop.
But what about other plants that are much smaller than fruit trees and may be much more tender. Plants like tomatoes, eggplant, green beans, potatoes (if they have not yet put up leaves they are fine), zucchini, cucumbers, etc., cannot take a freeze and must be protected or they will die. the good news here is if this does happen you have plenty of time to replant as for most of us in the freeze zone (zone 6b on north) should not have even been thinking about putting such crops out until late April at the earliest. So you will have plenty of time to replace the seedlings next month, you should even have time to grow all these things from seed (I just started peppers and eggplant, indoors, under lights today March 26). Most of the tender crops will get cold damage which will stunt their production usually by over 50% even with protection as they do not like it below 50˚F and even with protection they will be colder than that and while they won't die they probably won't be the same either.
But if you want to try and protect your tender crops than bed sheets, buckets and bushel baskets will be your friends here. If you have a lot of 3 to 5 gallon buckets put them over individual plants and the plants will be protected to about 20˚F, if you put a straw or leaf mulch in the bucket with the plants they are protected to around 15˚F, maybe even lower. Bushel baskets do the same thing as buckets. bed sheets are good for mass plantings or rows of plants. It is best to keep the sheet off of the plants by putting it up on sticks or chairs or anything that will keep the sheet elevated and not crush the plants. the sheet does need to hang down to the ground (cold air creeps along the ground and will infiltrate any openings near the ground and get inside the protection and freeze the plants), so put some rocks or bricks around the edges of the sheet(s) to keep them down.
Not everything needs protection. Early flowers such as daffodils and crocuses are very cold hardy and shouldn't be hurt by the freeze, even if they are in flower. Spring flowers like irises and tulips that have not flowered will not be harmed. But if they have flowered than they need protection and buckets are probably better than sheets. Crops such as garlic, onions, lettuce, spinach, peas and kale can take cold temps down to around 25˚F before they need any protection and than a simple bed sheet (or row cover if you have that) place over top should be enough protection. Spinach, onions and garlic are good down to around 10˚F before you get a lot of cold damage (which they will grow out of) or death of the plants with no protection.
I hope this helps you to save your plants in the case of cold after unseasonable warmth