Yes! You are very correct! This location was approved by the committee. The bench has been relocated. Mike needs to be removed and refreshed, he’s looking a bit peaked.
I am SO glad you asked, I LOVE to talk about this!
The fun part of the poison garden is how many of them are normal ornamental plants! In fact, the toxic potential of plants we consider benign is a huge aspect of the educational nature of the poison garden - the Goth/Wiccan connections are just an extra benefit.
Delphinium, foxglove (foxglove is the source of digitalis, an important heart medication with an extremely low safety margin), and lupine for example, as well as daylilies (extraordinarily toxic to cats, this particular plant has the only really toxic pollen in the garden, and it only affects cats who ingest the pollen…I usually post about lilies and cats on my Facebook page each spring as it’s insanely easy to cause complete kidney failure in cats with a single lily in a vase in your home). I may cage the daylily.
None of the pollen will affect any of the other garden plants (with the possible exception of cross pollination between the same species, and seed saving for next year - but that’s primarily tomato and pepper specific and applies to all garden plots in the space so not specific to the poison garden at all) and I checked with board certified toxicologists re: oleander, because there are rumors oleander nectar makes toxic honey and I didn’t want to poison any local apiarists or their customers! However, that isn’t true, because oleander does not even produce nectar. It’s funny how common this rumor is among beekeepers in the face of scientific data to the contrary. I even found a forum where it continued to be discussed as if it were real after someone posted a scientific paper describing the biology of the plant and the absence of nectar. One of the toxicologists is also a Master Gardener and she indicated the pollen is safe also for bees. Oleander is a common yard ornamental in California so they would have massive problems there if it were, especially considering that most commercial beekeepers travel to California every year to pollinate the almond trees. (there is a grayanotoxin that can accumulate in honey in Turkey - “mad honey” - but it is not a problem in the USA, which was also information provided by the toxicologists)
Rhubarb is also one of the poisons! The leaves are quite toxic, it’s only the stems we can eat, yet many folks grow this at home. In WWI, the British Home Office recommended citizens eat the leaves because ration supplies were running low - and they didn’t realize their mistake until people died!
Interestingly, tomato, pepper and potato plants are all members of the nightshade family and toxic on their own so they might make an appearance in the poison garden depending upon how space works out! If no space, we can have signage directing folks to the Master Gardener plot 
I am trying to stratify mountain laurel now but Bill Youngers doesn’t think it will grow well for us because it prefers a cooler climate. There will be a lot of plant test subjects! I sometimes say I have a Darwinian approach to gardening…if it lives I grow it again, if it dies I don’t 
The single most toxic plant is likely to be castor bean. I first saw this beauty at Botanica amongst all of their other gardens without any sort of warning, just a label! It was quite an impressive specimen. Castor bean is the source of ricin, but you have to ingest it, and the seed pods have to be chewed if they’re ingested intact. For a human it would take a fair amount. I have treated the poisoning in a friend’s dog who ate mole bait (that mole bait was whole castor beans, other mole bait is strychnine) - it looks just like parvovirus (vomiting/bloody diarrhea). I will be removing the seed pods so they can’t be accidentally ingested when Abby or other pups visit the garden.
There may be a plant or two which can irritate the skin if you touch it, but everything will have warnings. If I include hemlock it would have to be in a pot because it’s invasive. And of course, if one is nervous, the entire area is easily avoided which is why we selected a corner location!
Interestingly, the milkweed for the pollinator garden and the monarchs is also “toxic” - the sap being a skin irritant.
Aloe vera that grows in the lounge and also the ceramics studio is toxic as well! Ingestion of the Aloe plant latex is most known for it, but even topical use has been implicated in folks who are allergic to the lily family! Aloe - Mayo Clinic
Aloe vera: A review of toxicity and adverse clinical effects - PMC
The vast majority of ornamental houseplants are toxic. While many contain only oxalate crystals which are irritating, there are others such as Kalanchoe which you’ll find in Dillons and other local stores, in which some species contain heart toxins. Those plants don’t come with a warning either!
The poinsettia plant which nearly everyone “knows” is poisonous is not terribly poisonous either, primarily gastrointestinal irritant at adequate doses. Diterpenoid euphorbia esters and saponin-like detergents are the compounds of concern in them.
I’ve considered having a sago palm but haven’t sourced one. I won’t feel comfortable having a sago palm without caging it because dogs are very attracted to the leaves and it causes pretty profound liver failure in them. Sago palms (not a true palm, they’re really a cycad) are common house/patio plants in the USA also.
There are far too many interesting options I could include but won’t have space for. Some of the plants produce small amounts of cyanide when the plant are stressed - similar to apple seeds. For the most part, those primarily affect grazing livestock during droughts.
Access to the poison garden - the garden committee did not feel a fence was necessary. Bee and I have discussed a fence for the aesthetic aspect but we’re still working that out. Funds for the poison garden are limited because I’m not planning on using the Garden budget for it. Garden committee has been working on an access policy and authorization class for a couple of years now - the poison garden would be included in both when they are finalized, which is one of my goals for this year!
I also plan to host a class on the poison garden in summer once it gets going!