I'm sure a lot of people appreciate the fact that Stalagmites (called Dripstones in Renewed) cause damage, and can be used in various types of traps. However, rooting around underground for them may be rather time-consuming and painstaking. Making them farmable, and hence, renewable, would give the player an opportunity to organize better, as well as a sense of control over the process; yet, the farming process must not be overly easy. I propose a method of farming Stalagmites as follows:
1. Place a Stalactite on the bottom face of a Sulfur Ore or Nitre Ore block.
2. After a certain random amount of time, perhaps a few Minecraft days, a Stalagmite will form on the top face of the block directly below it, given at least one block of air between.
Most players could then place their found Stalactites on Sulfur or Nitre Ore that they discover while mining; so the cost of farming would be a combination of Stalactites, loss of otherwise obtainable sulfur or nitre, as well as the time spent finding the locations of your farms where you placed them. Of course, obtaining a Silken modifier to collect the ores for a more organized farm can also be a difficult endeavor. Thus, the farming process is not too easy, but it is simple, along the vein of vanilla Minecraft.
I appreciate the thought that you put into this to make it relatively accurate to real life. However, I'm not sure if dripstones are really used often enough that they would warrant the creation of an entirely new growth process.
Wouldnt it be simpler, and more logical to just add a crafting recipe for wooden spikes instead? Like 3 planks/logs on the bottom and 1 in the center. Boom, wooden "dripstone", causing the same damage, but craftable, and can match wood types for use atop wooden pallisades.