How you harvest depends on what you are harvesting and how you plan to use what you harvest. If you are adding to your salad or making a spaghetti sauce, you will harvest a few sprigs of what you need. Or you might be cutting the entire clump of certain plants a few inches above the ground. In my opinion, there are two important things to think about when you want to pick herbs: the time of day and the time of the season. The best time of day is on a sunny morning after the dew has dried. That's when the oil content is highest in the plants-I like to think of it as the energy of the plants is at its strongest. If you follow this rule of thumb, you'll pick when the flavor is best and the highest level of medicinal quality is available in the plants.
If you are harvesting leaves, peak time in the season is just before the plant flowers. That's because it takes a lot more energy to put out flowers than leaves and the energy of the plant is thus at its highest at that time of the year. When harvesting leaves, always cut the stems rather than stripping the leaves from the stems. Harvest only disease-free growth.
I harvest throughout the season as long as plant growth permits into the fall. Sometimes, time of the season for picking depends on the plants. You wouldn't want to wait until the end of the season to pick basil, catnip, parsley, or oregano. Why then you'd be eating Italian for a month straight and the cats would be jumping out of their skins. No, these herbs and others like chives, mints, sweet marjoram, sage, and thyme should be snipped as soon as they're established and are strong enough to continue growing vigorously. Be sure to trim off the flowers of basil as they appear to keep the plants from being flavorful and productive. At the end of the season and since it is an annual, harvest the entire plant before the first frost or a little before because basil really hates cold below 55°F.
Coriander and dill are a different story. They bolt quickly or go to flower and seed before you've overdone the salsa and chips and that can be a real disappointment. I stagger my plantings of these herbs so I'll have a constant supply throughout the season. They need to be picked as soon as they are 6 to 10 inches tall. Be sure not to snip the main stem of dill plants or they will stop growing. Just snip dill off the sides of the stems.
For my Thai dishes, I let lemongrass form 4 to 6 bulbous stems before I harvest it. I snip the coarse leaves anytime. My alpine strawberries are no let down for special desserts. I love them because they bear fruit all season long. When the fruit is ripe and ready for harvest, I know the leaves are ready to pick, too.
Rosemary is pretty woody, so when I want the stems to branch to make the plants bushier, I just snip there! Then I strip the leaves from the snipped stem and add them to my marinade for grilled lambchops. The same holds true for my scented geranium plants. I don't like them leggy, so I cut them back after each fifth or sixth node or growth area between leaves to keep them bushy. Then I use the leaves for tea or I dry them for potpourri. Speaking of potpourri, lavender is one of my favorites for making into sachets. I snip the flower stems before they open and the leaves anytime.
St. John's wort comes into flower the last week of June. I wait until the flowers are fully open to use in my infused oils. Later, in the late fall, I cut the entire plant to within 2 inches of the ground to use in tea or make tinctures out of the leaves. Some other perennial herbs like chives, oregano, and mints can be treated this way also in the late fall, but tarragon, thyme, and all the sages wouldn't survive the winter if I touched them in the late fall. They would just be stimulated to grow and then they wouldn't be hardened off in time for the big freeze. The best time for the final harvest of perennials should be no later than 1½ months before frost. If you are harvesting for seed, cut the stalks before seed begins to scatter.
Your harvest can be dried, frozen, or added to a preserving medium such as vinegar, butter, and oil.
Preserving by Drying
I'm talking about drying the leaves of the herbs here, which is the part of the plant that is most often preserved in this way. Most herbs dry well. That means that the herb does not lose its color, flavor, and aroma. There are a few that I never dry. Chives lose their taste. I don't freeze chives either because they are chewy when frozen. They are so easy to bring inside and taste so good fresh! Just make sure that you leave them outside to freeze before you bring them in. Dill dries just so-so. It loses flavor, in my opinion. I never dry cilantro.
I use two methods depending on how humid it is when I want to harvest:
Method #1: After picking, I hang the stems with leaves upside down or I put the leaves on a screen in a place out of the sun. You need to make sure there is good ventilation and low humidity to make this work well. When the leaves are crispy and crackly to the touch, they're dry. They need to be completely dry or they'll get moldy when you put them in storage. For herb seeds like coriander, hang bunches of seed stems in a bag to catch the seed. Dry in a warm, dark place with excellent air circulation.
Method #2: In the dog days of summer when the air is thick or when it's been raining a lot, I use the oven for drying. I spread the leaves out in a thin layer on a cookie sheet and bake them for 48 to 72 hours. If the stove is gas, the pilot light will keep the temperature hot enough. If you have an electric stove, put it on its lowest setting. Turn the herbs twice a day. Yes, it takes a long time, but a hotter temperature will dry the leaves too quickly and all the oils will be lost.
Storing Dried Herbs—The most important aspect of the drying is the storing. Never, and I mean never, crumble the leaves before you put them away. The less surface area exposed to the air, the better the herb will retain the oils you are trying to preserve. When you know they're crispy dry, strip them from the stems and store in air-tight containers in a cool, dry, dark place, and they'll keep their strength for about a year. Never store in sunlight.
Preserving by Freezing
This is easy. Just snip small pieces of the herbs and place in a zip-lock bag. Don't do anything fancy to them like blanching before you put them in the bags. They'll be good for about a year. If you prefer, puree the leaves in a blender with a little water to liquefy them. Freeze them in an ice cube maker and store them in freezer bags. For basil, I have a special way so it'll keep its color. I blend the leaves in a blender (you can use a food processor too) with a little olive oil. I pack the mixture into ice cube trays and freeze. When they're hard as a rock, I pop them out of the trays and put them in a freezer bag and freeze them that way. I've learned the hard way never to freeze pesto with the garlic in it. It becomes really bitter.