Lemon basil (istockphoto)

The end of summer always seems like the perfect time to make pesto. The basil has borne its fragrant leaves bounteously, but it’s starting to go to seed, despite our efforts to keep all the tips pinched to prevent flowers from forming. Quick! Let’s harvest armfuls of it and turn it into that luscious puree, to be frozen in little jars for a winter of pasta with pesto, omelets with pesto, and anything else that might cry out for the bright green taste of warmer days.

The other day I discovered some pitfalls along the road to pesto heaven, and I will never be blasé about making it again. My lemon basil, a favorite that had glorified many a green salad and tomato platter for months, needed to be yanked out in time for a sowing of fall carrots. So into the kitchen it went and from there into the blender with a large glug of olive oil. I wish I could say that I made this sauce with the traditional mortar and pestle, but a large batch such as this one would never be made if I had to go that long, arm-numbing route. I also omit pesto’s other ingredients — garlic, pine nuts and Parmesan cheese — when I make it in bulk, preferring to add them later and adjust them to the use at hand.

Before spooning my concoction into jars, I added some to a soupe au pistou I was making for a farm lunch. This is a soup flavored with pistou (as pesto is called in French) and composed of various summer vegetables — in this case green beans, shell beans, zucchini, tomatoes and garlic — all made more yummy by adding macaroni, browned kielbasa and shrimp. But when I tasted my soup, it had a bitter edge to it. What had gone wrong?

First of all, I’d broken one of the cardinal rules of cooking: Taste your ingredients before you toss them into the pot. Too late, I realized that my pesto was horrible, bitter beyond belief. I served the soup to my crew members, who ate every last bite with enthusiasm, bolstering my belief that enough kielbasa makes anything okay. But the failed pesto went out to the pigs.

I tasted the leaves of the fresh, pre-pesto leaves I had picked, and they were not that bad. Was it my cop-out with the blender? That had never been disastrous before, but perhaps a coarser puree, say, with just a few pulses in a food processor, would have been more mild. Olive oil can be bitter, though mine was fine. But maybe blending in a little water would have been safer, with the olive oil stirred in afterward.

Here’s what I concluded: that flavor compounds are mysterious and unpredictable, and something about this particular type of basil makes it especially prone to bitterness after it starts to form seed; that this is especially evident when the leaves are concentrated by the blender; that herbs are very powerful plants that need to be handled with great care; and that no matter how many times you have done something successfully in the kitchen, you should never lose your concentration. Keep your taste buds on full alert.

Damrosch’s latest book is “The Four Season Farm Gardener’s Cookbook.”

Tip of the week

Clumps of bearded iris should be dug and divided every three to four years to promote vigorous flowering. Use a garden fork to lift an entire clump and separate the rhizomes. Discard the old unproductive rhizomes. Plant the divisions at least 12 inches apart in a sunny and free-draining location. The rhizomes should sit partially above the soil line. Water them well but infrequently — overwatering will cause the irises to rot.

— Adrian Higgins