If you have a backyard garden, you know there’s no such thing as a single harvest time. You pick red tomatoes when they’re red, yellow ones when they’re yellow, lettuce when it’s salad-sized and eggplant. . . well, that’s a tough one, even for horticulturalist Melinda Meyers, author of dozen gardening books and host of the nationally syndicated TV and radio show “Melinda’s Garden Moments.” “Eggplant is a mystery vegetable — how do you tell when it’s ripe?”

The truth is, it’s not always easy to tell the best time to harvest the veggies in your backyard. This is especially true if your goal is to pick them when they’re at a nutritional peak. For example, salad greens lose all their vitamin C within two or three days of being plucked out of the ground. “Imagine how little vitamin C you’re getting when you buy greens at a supermarket,” says Meyers.

Related: Power Up Your Produce this Summer

But certain guidelines apply to all home-grown garden goodies.

Harvest in the a.m. Still-in-the-ground vegetables rehydrate in the evening and overnight, restoring moisture they lose in the light of day. They also turn the starches they produce during the day into sugar. Produce is sweeter, juicier and crisper in the mornings than later in the day.

Wait for a dry day. Pulling produce out of the ground when it’s wet increases your risk of food poisoning from Listeria monocytogenes, according to a Cornell University study published in the journal Applied and Environmental Biology. Wait at least 24 hours after irrigating your garden or a significant rain to pick vegetables. By then bacteria levels will have dropped to about baseline, the researchers said.

Related: 3 Ways to Eat Safer Produce this Summer

Pick only what you plan to eat soon. The minute you pluck a tomato or cucumber off the vine or pull a cabbage or head of lettuce from the soil, its quality and nutritional value starts to degrade rapidly. Vitamin C is particularly fragile. Peas, for example, lose about half their C 24 to 48 hours after picking.

Related: 6 Ways to Get Your Kids to Eat More Veggies

Get them at their peak — not beyond. You may win the county fair award for biggest cucumber, but that cuke might not be too tasty. Many vegetables develop tough flesh, hard seeds and bitter flavor as they grow and age. Cucumbers are at their best between 6 and 10 inches long. So are zucchini. “The goal is not baseball bat size,” says Meyers. 

Here’s what she says about prime-harvest times for other picks.

  • Eggplants: An eggplant that’s large, shiny and retains the indentation from slight pressure of your thumbnail is good to go. “If it bounces back it’s overripe,” she says.
  • Peas and beans: These should feel firm and look full; they shouldn't be yellow and bulgy. At their peak, beans will snap sharply and peas will taste sweet.
  • Broccoli: Heads should be 3 to 6 inches in diameter, bluish in tint and flower buds should be closed. Don’t let broccoli sit in the mid-to-late-summer heat. “Once hot weather sets in, buds will break open and the flavor won’t be good,” says Meyers.
  • Carrots: Pull carrots when the root tops are ¾ to 1 inch in diameter.
  • Cabbage and head lettuces: Harvest them when they look like the ones you buy in the store. Pick lettuces before hot weather turns them bitter. Remove the outer leaves of leafy greens like chard, spinach and kale so they continue producing. “Take a bucket of water with you when you’re picking leafy vegetables,” says Meyers. “If the leaves start to wilt, you’re starting to lose vitamin C.” 
  • Peppers: Allow them to ripen on the plant — they stop ripening once they’re picked. Most green peppers will turn into red peppers, a sign they’ve produced lycopene, a red pigment that’s also a powerful antioxidant linked to lower risk of cancer, heart disease and eye disorders.
  • Green tomatoes: Pick them before the first frost, but note that even if you allow them to ripen indoors (which they will, in a paper bag), they may be less sweet and may not have as much vitamin C as those that ripen on the vine. They may still be rich in lycopene.

Denise Foley is a veteran health writer and a former contributing executive editor at Prevention magazine.