I depend on herbs all year round for my cooking and freezing is the best way to preserve some herbs that I grow. I find they can be kept for at least a full year. The flavor is far superior to dry for selected herbs. It is also quite convenient to use a machine to process a batch at time instead of hand chopping small amounts as needed.
This is the method I use.
For Italian Flat Leaved Parsley:
Pick only the outer leaves as they mature and the plant will continue to grow from the center in a rosette fashion producing new leaves until hard frost. Parsley is a biennial and if the plant is left in the ground it will produce new leaves early in the spring. Once the weather is warmer the 2-year-old plant will go to seed and the leaves will turn bitter. It can be removed at this time OR better yet, left to go to seed. This will set up a new crop of volunteer baby plants that will start the cycle again. Since parsley is notoriously difficult to germinate in pots this is a big advantage. If seedlings are to be transplanted it should be done while they are quite small as they form a taproot.
Make a cluster of the stems and swish in water to wash. Shake gently to remove excess water. Trim ends and stand in a small glass of water and allow the leaves to air dry.
To process and preserve: Trim dry leaves off stalks and chop finely. I use a food processor on “pulse” - be careful not to puree the leaves. Freeze in zip lock bags (or use "Press and Seal" saran wrap)in a thin layer. Squeeze out air before freezing.
Stems can be frozen separately and used in making broths.
For Sweet Basil:
Lower leaves frequently get tough as they age. Remove them as soon as they reach full size. Plants go to seed when the weather is warm and the plant is mature. Basil is an annual and when annuals go to seed the plant dies. To prevent this and to keep the plant productive cut off seed heads as soon as they begin to form. If young and tender process with the leaves. This deadheading will also force the plant to branch out which means more basil for later.
Harvest can be of entire branches or individual leaves. If stems are available wash and stand in water as with the parsley. If only leaves are being processed wash them and lay them on towels in a single layer to blot dry. Process as with parsley if you plan to use chopped. Do not refrigerate, basil will turn black when cold. If quickly frozen it will maintain its green color. When cold weather is approaching harvest all your basil or it will turn black in the yard.
Basil can also be frozen in small clusters of branches with leaves still attached to the stalks. These should be the size that will be used in a single recipe. I like to use these in sauces where I want to infuse the flavor of the herb but not have green flecks floating about. I cook with them then remove the branch before serving. It is fine to cook with the stem attached.
Basil can also be prepared as pesto and frozen in various sized containers from ice cube trays to 1 cup or so. It can be used as a sauce by itself or as an ingredient in other dishes.
Leaves should be cut at ground level while still young and tender. Once flowers form the leaves become tough. The flowers can be allowed to bloom as a decorative accent, then the entire plant can be cut down to ground level to revitalize. New tender growth will come again 2 or 3 times a season.
Leaves can be left long or trimmed into little circles with a pair of scissors. These can be frozen in zip lock bags (air squeezed out of course) (or use "Press and Seal" saran wrap) or in cubes of ice in order to preserve the nice shape. If ice cubes are used allow melting and draining before using for baked potatoes. If used in soup ice cubes can be placed directly in.
Cilantro, dill, fennel, mint, all freeze nicely as well. Oregano and thyme seem more suited to drying. Rosemary is just best fresh, I'm not happy with the frozen nor the dried result. Happily one doesn't need much. I plan to just bring the plant indoors this winter. It will likely die before spring but I'll have fresh rosemary a bit longer and start over in May. C.L. Fornari recently recommended the rosemary variety Salem as being less problem inclined when grown indoors. The variety Arp is very cold tolerant. Alternatively, one can cover Rosemary with leaves and a bushel basket in the coldest part of winter and it will likely survive most (zone 7 at least) winters allowing any harvesting one is brave enough to undertake in the cold.
Rosemary is now known as Salvia rosmarinus instead of Rosmarinus officinalis. This reflects a change of genus in 2019 recognized by the RHS following "research" which exact nature I do not know but suspect is genetic.