Add on top of that a lead "kenzan" and an arrangement that uses heavy extending branch material and you have greatly increased the possibility of the whole thing tipping over and potentially breaking your lovely container.
The solution to this dilemma is to lower the center of gravity and make the container + kenzan + arrangement less top heavy - but how? The answer is to simply increase the weight at the foot as much as possible. The more weight present at the foot of the vase, the lower (more stable) the center of gravity.
So how do we do that? How do we make the bottom third of the container weigh more than the top two thirds plus the added weight of the water, kenzan and arrangement? Many Ikebana designers use decorative rock in the bottoms of their containers to add more weight. Decorative rock and pebbles can be useful, but they are only about twice as heavy as the same volume of water. What's the best thing to use then?