Pumpkins: Pick when fruits are full size, the rind is firm and glossy, and the portion of the pumpkin touching the soil is cream or orange in color.
Radishes: Pick when 1 to 11/2 inches in diameter.
Rhubarb: Pick when stalks are 8 to 15 inches long. Flavor and tenderness are best in spring and early summer. Harvesting from well-established plants may be continued throughout the season; may want to pull all leaves present just before the first fall frost.
Spinach: Pick when larger leaves are 6 to 8 inches long. Pull larger, whole plants from the row until you harvest all plants. Spinach that is planted in early spring goes to seed when the days get longer. If spinach is planted in early August, it does not usually go to seed during the shorter days of fall.
Squash: Pick winter squash when it is full size, the rind is firm and glossy, and the portion of the squash touching the soil is cream to orange in color. Pick summer squash when 6 to 10 inches long.
Sweet Corn: Pick when kernels are fully rounded, but still filled with milky juice. Harvest about 21 days after silk appears. Pull each stalk once the last ear of corn has been harvested.
Sweet Potatoes: Pick in late fall, but just before the first early frost. Make sure to dig up carefully to avoid cuts, bruises, and broken roots. Use smaller, younger roots soon after harvest, as sweet potatoes typically do not store well.
Tomatoes: Pick when fruits are fully colored. For fully ripe tomatoes, leave completely red fruits on healthy plants for 5 to 8 days during the warm, sunny days of August and very early September. Pick only fully ripe tomatoes for juice or canning to ensure full flavor, good color, and maximum sugar content. Tomatoes will ripen indoors if picked at a mature green stage or when some color is showing.
Watermelon: Pick when watermelon is full size, dull in color, and the portion touching the soil turns from greenish white to cream. The tendrils nearest a melon will curl and dry up when a melon is ripe.