They would decline if pot bound. But since yours are sunk in the ground they're probably rooting through the drain holes and getting nutrition that way.

Hello - it was me with the pot bound lilies. I grew them in pots to protect them from the voles too. After about five years of growing in pots my lilies multiplied and were so tight in their containers (10, 12 and 14" plastic pots) I couldn't water them from the top anymore because the soil in the pots had become completely full of roots and bulbs - the soil was pushed up to the very top of the containers. They had to be watered by submerging the pots in a tub and it became a hassle. Some of my potted lilies were very good multipliers, one bulb became many bulbs and last fall when I unpotted them to plant them in the garden the plants were so tight in their pots that the pots had to be cut off of them to get them free and into the ground.

I have paid close attention to my gardens for over 20 yrs here and have found several places where the voles leave my lilies (and tulips) alone. Voles don't like open exposed areas, they want some kind of cover to hide and dig in. Keep the weed growth down and mulch only sparingly. The back side of my lily bed is the edge of my driveway, the front and sides are lawn grass. My lily/tulip bed is at the highest elevation of the surrounding area which voles don't seem to like either.

Another reason I like to grow lilies in pots is because most of the lily bulbs that are sold are on the small side and it takes a couple of additional years for them to mature. Growing them in pots seems to speed up the process because the plants are getting water and fertilizer on a regular basis.

If you have lilies growing strongly in the same pots for five years or more they probably need to be put into larger pots. If you sunk the pots in the ground they have most definitely grown roots out of the bottom (and probably the top) of the pot. Having a lily growing in a sunken pot isn't enough to protect it from voles, I have had voles chew through thick plastic pots to eat what was growing inside. To add insult to injury they made nests and raised families in the pots! Metal hardware cloth or other physical barriers are the only way to keep them out if you have voles in your garden, or keep the pots of lilies growing sitting on the surface of the ground, not in the ground. The best time to dig and divide lilies is in the fall when the top growth has died back and the plants are dormant.

Good luck with your lilies!

@Jeb zone 5..Another vole or pocket gopher(I have BOTH) deterrent...is to pour something organic but nasty smelling into the ground,bordering your lilies.

Ie: White vinegar

*A few drops of lemon oil or cinnamon oil in a gallon of water

I found a GREAT deal on tall red lilies if anyone is interested: Home Depot, Shipping is FREE!

https://www.homedepot.com/p/Garden-State-Bulb-14-cm-16-cm-Asiatic-Lily-Red-Velvet-Flower-Bulbs-Bag-of-10-ECS-38-10-01/325004407