Roughly chop the flowers, leaves and stems of the rose geranium and place in a large glass bowl with the lavender and marigold. Bring 3/4 cup water to a boil and pour it over the flowers to make an infusion. Let infuse until the water has taken on the color of the flowers, 10 minutes. Transfer the flowers and infused water to a blender and whiz. Strain the mixture through a piece of muslin into a clean bowl. Dissolve the gelatine in 2 tablespoons of cold water in another bowl. Gradually add the flower infusion, stirring to separate any lumps. Add the vodka and tea tree oil, stirring until a gel is formed. Pour into a pot with a pump dispenser using a funnel. Apply to affected areas 2 times a day, or as frequently as needed. The gel will keep in the refrigerator for up to 6 weeks.