Thoroughly scrub cucumbers with a soft vegetable brush in plenty of cold running water.
Remove stems and blossoms; slice off each blossom end. Cut cucumbers lengthwise into quarters.
In a 4- to 5-qt. stainless steel, enameled, or nonstick pot combine water, vinegar, sugar and pickling salt. Bring to boiling, stirring to dissolve sugar.
Pack cucumber spears loosely into hot, sterilized pint canning jars, leaving a ½-inch headspace. Add 1 tablespoon dillseeds to each jar.
Pour hot vinegar mixture over cucumbers, leaving a ½-inch headspace. Discard any remaining hot vinegar mixture. Wipe jar rims; adjust lids and screw bands.
Process filled jars in a boiling-water canner for 10 minutes (start timing when water returns to boiling).
Remove jars from canner; cool on a wire rack. Let stand at room temperature 1 week before serving. Makes 6 pints.