A vast array of flavorful herb and spice adjuncts have been used in mead making. Herbs are typically the leaves of plants and can be used fresh or dried. Spices can come from any part of a plant except the leaves: bark, seeds, roots, etc. Spices are typically dried or cured in some way.
Some plants provide both an herb and a spice because we use their leaves and some other part to flavor food. When this is the case, the same plant may have two different names used according to the part being referenced. One example is cilantro/coriander; Cilantro refers to the leaves, coriander refers to the seeds.
A mead made with herbs and/or spices is called a "metheglin." This is one of the major categories into which most meads fall. It is a category both broad and deep and includes disparate styles such as capsicumel and tej.
As with all ingredients, quality and freshness matter. While there is nothing wrong with using grocery store spices, you can get better results procuring spices through a specialty shop. A great spice store will have no problem allowing the customer to smell sample everything in the shop (within reason).
Whenever possible, try to purchase herbs and spices as close to the time you plan on using them as possible, from a reputable source that has enough traffic that their stock doesn't sit around. Just because it's new to you, doesn't mean that it hasn't been sitting on a shelf for months or more.
Lastly, buy whole spices when possible. Whole spices retain aroma and flavor better than those that have been processed.
Store your spices and dried herbs in a cool, dark, and airtight environment, and in the most complete form you can.
Fresh herbs can spoil quickly. If you can't use them quickly, take steps to preserve them. Herbs can be frozen, dried, or infused into an alcohol or oil tincture.
Process (cut, slice, chop, grind, grate, crush, etc) your herbs and spices as close to adding them as you can. This will retain as much of the volatile flavor compounds and oils that hold the flavors which are the whole point of adding herbs and spices. When cutting or chopping herbs, use a sharp knife or scissors to avoid bruising and tearing.
Herbs are frequently (not exclusively) used in their dried forms for mead making. Consider using a tea infuser or tea bags. A tincture can be made with with strong alcohol. Vodka or Everclear provide a neutral flavor. Strongly flavored liquor like bourbon may also have desirable characteristics depending upon the recipe and desired flavor profile.
When added directly to the fermenter without a tea bag infuser for containment, dried herbs will often facilitate the formation of a krausen on the surface of the must. To avoid overflow, ferment in a bucket. It's a good idea to regularly stir it during the first third of fermentation to re-suspend particles and keep the flavoring adjuncts in contact with the must.
Hot and cold steeps can yield different flavors. To hot steep, first make a large tea then mix that with the honey instead of water.
The mint family
Tea: white, black, or green.
Flower petals and parts: rose hips, rose petals, hibiscus petals, chamomile petals
Spices come from every plant part except the leaves and therefore come in an endless variety of forms. Whether a spice is added whole or processed in some way, they can be added during or after fermentation. It's a good idea to start with less spice than you think you need and slowly step it up until you reach the desired flavor intensity and profile. When possible, use a sanitized tea infuser, tea bag, or mesh bag to contain the spices you're adding. Attach and run a piece of sanitized string (fishing line works great) out of your carboy. This makes them very easy to remove later without racking.
Ground spices must be used with care. Flavor extraction will not be instantaneous and the tiny particles are not easily removed. This makes it easy to overshoot with spices like ground cinnamon, clove, allspice, etc.
Some spices come in seed pods or with tough outer shells. It's a good idea to crack or grind these with a a mortar and pestle or coffee grinder before use. Consider toasting your seeds and pods before grinding or cracking them. These are marked below with a dagger (†).
Root spices, if available fresh, should be cut or sliced thin for flavor extraction. Otherwise, they probably need to be ground.
Common Spices (Alphabetical)
All-spice†: Actually a dried, unripened fruit. Crack for better flavor extraction.
Cardamom (white, green, black): These are seed pods from the same species of plant. White & Green are harvested before the pods reach full maturity (white are harvested as green and bleached). Black are harvested at maturity and then dried. Green pods are typically used whole or cracked. Black usually get cracked open to remove the seeds which are then used; the husks are discarded.
Chili Peppers can be used fresh, dried, or smoked, whole, ground, chopped, or de-seeded. Capsaicin, the compound that gives peppers their heat, is concentrated in the white pith membrane to which the seeds are connected. The seeds themselves do not produce any capsaicin themselves. If you want less heat, clean out the pith.
Citrus Zest (dried orange peel): The colored outer portion of citrus peel holds aromatic oils and flavor compounds, but the white pulp beneath (the pith) is extremely bitter. The easiest way to get fresh zest is to buy fresh oranges and remove the zest yourself with a citrus zester, though a small cheese grater can work with care. Dried sweet orange peel is also available commercially and may be used as-is.
Cinnamon: There are several varieties of cinnamon. Listed below are the two major varieties you should recognize. It can be used in ground or stick form. Sticks ("quills") are easy to add and remove.
Cassia: Most Western grocery store cinnamon will be Cassia, which is very intense and slightly "hot." This is the cinnamon that gives Big Red gum its powerful bite. It's also probably the variant you want to use to mimic western apple, peach, or pumpkin pie flavor profiles.
Ceylon: In other parts of the world, Ceylon cinnamon is more popular. It's sometimes called "true" cinnamon, but that doesn't mean it's better for every application. It has a sweeter flavor profile and lacks the bite of Cassia.
Clove: A little goes a long way, even when using whole cloves. Even a single whole clove can strongly flavor a gallon of mead.
Coffee: When used in primary, coffee can add vegetal flavors. More control can be had by making a coldbrew and adding it to-taste post fermentation. To do this, boil the water for your coldbrew for 15-20 minutes (to drive off oxygen) before you chill and coldbrew. Use as little as possible to avoid diluting ABV; coffee's pH is usually above 5, causing pH, and therefore sulfite requirements, to increase. Use too much, and you'll have to use acid blend to restore a lower pH.
Fennel Seeds(†): These tread the line between sweet and savory.
Ginger (fresh or dried): Fresh Ginger should be sliced into thin slices or matchsticks. Dried ginger is usually available ground.
Juniper Berries†: Overuse can make your mead taste like gin. Don't harvest these yourself unless you're sure you know what you're doing. Some varieties are carcinogenic and poisonous.
Licorice: A little goes a long way.
Nutmeg: Available pre-ground or as a whole seed. Whole seeds require patience or special grinders to use effectively.
Peppercorns†: Crack or grind for better flavor extraction.
Star Anise†: Typically used whole, but can be broken, cracked, or crushed. A little goes a long way. Flavor similar to licorice.
Vanilla Bean: If adding directly to your Must, split the beans lengthwise. When making a tincture over several weeks, you can use it whole, but extraction will take longer.