Expert: C.L. Fornari - 8/14/2008

Question
I live in central California,yes it's really hot here now.I planted a hydrangea in a varily large pot with a drain hole in the bottom and a dish to cstch the water. It sets outside by my front door and is always in the shade. It has been doing fine blooming, nice full foilege etc. It is watered on an individual basis with a automated watering.It watered 3 or 4 minutes every other day. We have not changed that but all of sudden we noticed that the water was just filling up the pot and then staying there. Some water would leach through the hole but the water stayed at the top of the pot. We stopped watering, poured the excess water out and the plant slowly died(all shriveled) but the soil is still very wet and we haven't watered it(nothing to water) in over a week. I want to put another hydrangea in this pot but not until I figure what happened. Do you have a clue? Appreciate any help.Oh the soil was replaced when I planted this bush with regular potting soil. thanks Lou

Answer
Lou,
Two things were probably in play here - as a plant's roots fill the pot the soil tends to first get pushed aside a bit and next compacted toward the bottom of the pot. The roots grow in volume and the dirt has no where else to go - this is especially true if you have a shrub or tree in a pot since these have bigger size roots and root systems.  If at a certain point the soil clogs the drainage hole, the pot holds the excess water and then the roots begin to rot. At first you don't notice anything wrong because the plant is still coping, but as the roots rot the soil stays even wetter because the roots aren't drawing the same volume of water up and away from the soil. And because the roots are rotting and not giving the water to the leaves, the foliage shrivels and dies.

This doesn't always happen, of course - sometimes enough soil leaks through the drainage hole or gets pushed aside so that all proceeds without "sudden death."  Potting soils high in peat moss might be worse because they don't drain as well, and perhaps your soil was very "peaty" - potting soils differ in quality and if you notice when you buy a container grown shrub or tree it's seldom planted in a peaty soil - commercial growers use soil that is pretty sandy in their containers so that this won't happen.  They also leave the soil pretty loose when potting up so that there are plenty of air spaces for the roots to grow into.

At this point you could go two different directions: you could pot up another one using a good potting soil that is NOT high in peat - you could even mix some sand into the soil or use cactus mix - and be sure not to press the soil down when you plant the shrub - leave air spaces in the mix.  OR you could pot the plant in a nursery pot that just fits into your other container - nursery pots have at least four drainage holes on all sides so that the water is sure to drain well - pot up the plant in the nursery pot and then set it in your more decorative pot and water with the system you describe as long as the weather is hot - we have the take the "hydra" part of hydrangea seriously in hot weather!

I hope this helps,
C.L.
C.L. Fornari
www.gardenlady.com