I have 10 half barrel planters that I have been trying to establish for two seasons now. I also have about 15 plants in the ground which are well established and producing, so I have been able to take note of the differences between container gardening and ground gardening.
My first issue was with drainage. Although I drilled holes in the bottom of the barrels and layered each with gravel (something I am now told actually hinders proper drainage...?) I did not anticipate that I would need some space beneath the barrel to allow air to flow to the lower soil. a few 2x2 wooden spacers helped a lot.
I encountered another problem last summer with the soil being too compacted for the young root systems to get through. I had to dig them all up and amend the soil with a bit of peat moss. This seemed to have helped, though some of the plants have yet to really take hold.
At the end of last season after cutting down the bines I topped up the containers with new soil (soil will both compact and leak out over time) and a thick layer of mulch thinking it would be smart to provide some additional insulation for the Colorado winter (Front Range winter, so pretty mild actually).
As far as nutrients, I find that my container plants can utilize more frequent feedings than the ground plants. For both gardens I use a fish fertilizer from home depot...its mild and balanced and only slightly higher on the nitrogen.
This year my goal is to nail the pre-flowering nutrient additions. In the past I have added small amounts of bone meal as flowering begins but I have not noticed it having a significant effect; I am told by a gardener of a different variety that a more specialized fertilizer would do the trick. Anyone have suggestions?
As far as watering, I probably did it way too much in the first season. Everyone likes to talk about how much water hop plants drink, and although it's a valid fact it just leads to wasted water and soggy roots. In the second season, I paid close attention to the moisture level in the containers and only watered when all the soil had dried out. This technique is supposed to encourage the little rootlets to stretch and grow to find water. What i found was that the barrels were actually quite good at retaining moisture and I only needed to water once per week (a good soaking).
I am looking for more advice on growing in large containers if anyone has any. I'll try to get some pictures up too. Cheers!