The quickest way to kill a geranium is to drown it, drainage is everything!
Remember, when you are growing geraniums, you are, for all intent and purpose growing cacti and succulents. Geraniums MUST have good drainage and a part of that is provided by the soil mix you use, but in reality, it is regulated by the drainage holes that are in the container in which you choose to grow your plant.
Most plastic pots have more than ample drainage holes or slots and it is usually the terra cotta pots that have the more restricted hole sizes. I generally like to see a 5/8 inch diameter hole in a pot as small as 4 inches. If you are using elongated terra cotta pots or shallow ‘bonsai’ pots for species pelargoniums or sarcocaulon you may need to have more than one drainage hole.
To enlarge or to make additional holes I use a 3/8 inch electric drill with a 5/ 8 inch masonry bit and it quickly sovles the problem. After you are satisﬁed with the drain holes in the plastic or terra cotta pot, the remaining issue is keeping the soil mix from washing out through the drainage holes.
Some time ago I made the decision to use a soiless potting mix for all of my geraniums excepting the species pelargonium, monsonias / sarcocaulons, and erodiums. The potting mix that I use has a great amount of coarse material but it also contains ﬁne materials and a gritty sand.
I do not want the material to wash out of the pot through the drain holes during normal watering practices. The planting mix used by growers varies greatly and the problem of soil loss can be signiﬁcant. I recommend the use of wire or plastic mesh / fabric to cover the drain holes or slots in the pot as use of this material does not lesson the space for soil within the container. (see illustration)
If you use a tight mesh which has small openings you may ﬁnd that the soil tends to plug up the holes in the mesh. This will greatly reduce the drainage efﬁciency and will lead to a waterlogged soil condition and sure death for your prized geranium.
Let common sense prevail.