And it shouldn't have been fertilized with chemical fertilizer in, say, the past year. I've found that the hardest part to making flower essences is finding flowers in the best possible condition. It's more of a challenge to find small individual plants like SJW or comfrey, because even if you know someone who has an herb garden, usually they have only one or two plants, or the flowers are very small, or one batch of flowers wilts before the next comes into bloom, etc. etc. etc. And places that grow fields of herbs commercially almost always use chemical fertilizers. Fortunately, it's not too hard to grow a bunch of small herbs if you have a little garden space and can plan ahead.
Roses take a little more work, but they're nice to have around anyway.
Here is a brief summary of the main uses of the ones I have references for. The Gurudas book contains much more detail, but even it can't list every flower in the world. Treats mass hysteria or panic, such as that following disasters like earthquakes, fires, floods, the arrest of a national football hero for murder, the parking situation during the World Cup, etc. Augments psychic abilities, especially the exchange of systems of philosophy between East and West. Treats a variety of diseases that result from the suppression of psychic abilities, including allergies, asthma, colon spasms, epilepsy, Meniere's disease, migraine headaches, mucous colitis, chldren's night terrors, obsessional neuroses, and duodenal ulcers.