It's really hard to pick a perfectly ripe watermelon — unless you use these expert tips

Summer is filled with mysteries. Case in point: Why can some people consistently pick the most flavorful, crisp watermelon for every picnic or potluck, while I invariably choose clunkers — mealy and mushy — that seem to contain notes of sawdust?

This is why I asked melon grower Jeff Nistler to show me around his Maple Plain farm where he tends to five varieties of watermelon and nine kinds of cantaloupe. If anyone could detect a delicious melon, it's Nistler.

Rather than scoff at my ignorance, he sympathized with me — and anyone who's resorted to sniffing or thumping their produce in a supermarket.

"The consumer is at quite a disadvantage when selecting a melon," he said.

That's because some of the telltale signs of ripeness that growers rely on are no longer apparent by the time watermelons arrive at the grocery store, or even at the farmers' market.

In the field, the first thing Nistler inspects is the melon's tendril, the little curlicue on the plant. It must be dry and brown, signaling that the melon is ripe.

But the humble melon shopper never sees this signature clue in the store. Not only is there no tendril, but the entire selection of melons, to the untrained eye, may appear flawless.

"In the store, things are produced to look perfect and ship," said Nistler, who sells his produce at the Mill City Farmers Market in Minneapolis.

The other way he knows a watermelon is ready? He cuts and tastes it. (Don't try that in the produce aisle.)

Standing in his tall rubber boots beside his UTV, Nistler used a kitchen knife to halve a perfectly round melon over a makeshift cutting board. When the blade cleaved through, the melon produced a popping sound — another sign it was ready. A gorgeous red flesh appeared.

As Nistler took a bite, the grin that emerged on his face was almost the size of his giant watermelon wedge. He offered a slice to me, and for a moment there was no place I'd rather be than among his rows of melon, devouring that sweet decadence.

"Don't you feel a certain kind of happiness? Anything that ails you, a melon is good for," Nistler said, only half-jokingly. "You know how everyone is into the edibles? It's like an edible, without the impairment. It can change your mood if you're suffering."

The prime season for watermelon picking in Minnesota is mid-August through Labor Day, maybe a week or two after, depending on the season, he said. For those of us who can't sample melons in the field, here are some tips for purchasing this picnic table staple — just in time for the last few summer gatherings.

Look for the field spot

This is Nistler's top piece of advice, by far. When a watermelon rests on the ground, it develops a field spot. For typical melons that have the green rinds with jagged stripes, the field spot should be yellow, rather than white or a pale green, for optimum ripeness. For melons that are more solid and dark on the outside — like the Black Diamond variety — the field spot should be a deep yellow, turning orange.

"You wish you could have a spectrometer to see into (the melon), but you can't. So it really comes down to that yellow spot," he said.

Feel it in your hands

My Star Tribune colleague food writer Joy Summers tells me that a good melon should feel heavy for its size. Pick up the melon and compare its weight to others roughly the same size. I use this trick with other produce, especially oranges, but it holds true for melons, too. The denser it is, the more water it likely holds. And who doesn't love a juicy melon?

Tap it

A young vendor at a roadside farm stand once told me the tastiest watermelons should sound hollow. "It's counterintuitive, right?" he said. He even knocked on several for me, as if playing the marimba, before picking the most hollow-sounding. (And yes, it was delicious.)

Nistler cautioned that the sound isn't as important to him as other indicators, but that often a good melon will typically sound "tight like a drum."

Smell it

For cantaloupe and musk melon, don't be afraid to put your nose right up to the stem end. (This trick doesn't work for watermelon.)

"You should be able to smell a sweetness to it," Nistler said. "If it doesn't have the aroma, it's not ripe — or it's not good."

That doesn't mean it doesn't have the potential to become ripe. If the scent isn't there, let the melon sit on your counter for a day or two. "You'll know for sure when it's ready," he assured me.

Nistler says that on a recent weekend, most of his customers were smelling the wrong end of the melon. The part that was attached to the plant's flower may smell like dirt, but the stem side will carry that sweet scent if it's ripe.

Buy direct

If you purchase produce at a farmers' market, growers can guide you to the right melon — for your gathering in two days or for immediate gratification.

They also can introduce you to a world beyond the typical supermarket varieties. Nistler's most fervent cantaloupe customers know the difference between a sour melon (Melonade), a refreshing variety named after a toad's skin (Piel de Sapo) and a melon brimming with sweetness (Sugar Cube).

Many reputable growers stand by their picks.

"Because the buyer is at such a disadvantage, we guarantee the melons," Nistler said. "We replace or refund if we're wrong."

As a shopper, being wrong on a melon isn't the worst thing to happen. But when precious summer is fleeting, I'd rather leave little to chance — and trust the professionals.