The spell calls for mugwort. You do not have mugwort, the shop wants twelve pounds plus postage, and every substitution list online tells you to use something else that "also means dreams."
That method is why substitutions feel arbitrary and why they often do not work. Here is a better one.
Substitute by function, not by keyword
In any spell, each herb is doing a job. Work out the job and you can replace it properly.
The actuator. The engine. Usually hot, sharp or stimulating — cinnamon, ginger, black pepper, chilli. It makes the working move fast. If your spell has one, it is usually the smallest quantity and the most important.
The director. Carries the intent — this is where the correspondence actually matters. Rose for love, bay for success, rosemary for memory and protection.
The carrier. Bulk and body. Often neutral and often overlooked. Oats, rice, salt, plain herbs, dried flowers.
The protector. Keeps the working clean and stops interference. Rosemary, rue, salt, nettle, thorns.
The sweetener. Softens and draws. Honey, sugar, vanilla, apple.
The fixative. Holds it in place. Resins, roots, salt, knots.
Now the substitution question becomes answerable. A missing actuator can be replaced by any other actuator you own — ginger for cinnamon, black pepper for chilli. A missing director needs something else pointing the same way. A missing carrier can be almost anything inert.
Swapping a director for an actuator, which is what keyword lists routinely have you do, changes what the spell is.
If you cannot work out which job your missing herb was doing, ask in our live chat. Tell us the spell and the ingredient and we will tell you what it was there for and what replaces it.
What is already in your kitchen
Nearly every household has a full working apothecary and does not know it.
Rosemary — the great all-purpose one. Protection, cleansing, memory, clarity. If you learn one herb, learn this. It substitutes for a surprising number of protective and clearing ingredients.
Bay leaves — wishes, success, banishing. Write on them and burn them. In every supermarket.
Salt — cleansing, protection, fixing. The single most used material in folk magic worldwide.
Black pepper — banishing, protection, heat.
Cinnamon — speed, money, heat, passion. The classic actuator.
Basil — money, love, peace in a household.
Thyme — courage, cleansing, sleep.
Garlic and onion skins — protection, absorbing.
Coffee grounds and tea — speed, grounding, clarity.
Lemon — cleansing, cutting, clearing.
Sugar and honey — sweetening.
Vinegar — souring, banishing.
What grows free
If you can walk somewhere green: nettle (protection), dandelion (wishes, divination), plantain (healing), clover (luck), elder (protection, and treat it respectfully), pine and cedar (cleansing), rose (love), yarrow (courage, boundaries), mint (money, clarity).
Learn to identify anything before you pick it, and do not forage where dogs walk or beside busy roads.
When you have nothing at all
Write the herb's name on a slip of paper and use the paper.
This is not a sad compromise. Name magic is old and it is used deliberately in several traditions. The written name of a plant functions as the plant in a working, and if your whole practice is a candle, a pen and paper, you have a complete practice.
When you should not substitute at all
Three cases where finding the real thing matters.
When the herb is the entire spell. Some workings are built around one plant and its specific relationship to a tradition or a spirit. Substituting there does not weaken the spell; it makes it a different spell.
When a tradition specifies it. If you are working inside a named tradition that calls for a particular plant, that requirement is part of the practice and swapping it is a decision to work outside the tradition. That is allowed — just know you are doing it.
When you are learning. Do a working once as written before you start adapting it. You cannot tell what a substitution changed if you never saw the original.
Outside those three, substitute freely. Folk magic has always been practised with whatever was in the house, and the idea that a spell requires an exact and purchasable ingredient list is largely a modern invention of people selling ingredient lists.
The safety part
Substitution lists never mention this and they should.
Mugwort — the herb people most often want to substitute — is an emmenagogue and is contraindicated in pregnancy. So are rue, pennyroyal and tansy. Pennyroyal in particular is genuinely dangerous and should not be used internally at all.
Rue is phototoxic on skin in sunlight and causes real burns.
Do not burn indoors: chilli or cayenne (the aerosol is genuinely harmful, especially with asthmatics, children or pets in the house), anything you cannot identify, and any oleander or yew, ever.
Toxic to pets: many essential oils around cats, and a long list of common plants around both cats and dogs.
Magical use does not suspend chemistry.
Ask before you swap
Working out which job an ingredient is doing is easy once you have seen it done and awkward the first few times.
Open the live chat at the bottom of this page, tell us the spell and what you cannot get, and we will tell you what that ingredient was actually for and what in your kitchen does the same job.
It takes a minute, it is free, and you will almost always find the substitute is already in a cupboard.
Or browse our full range of workings.