Ground squirrels digging tunnels under avocado trees

Hi permies folks,

Thanks in advance for the help. I just started to help with the managing of an avocado orchard, ~1,200 trees. ~12 acres, ~5 years old, planted in off-contour rows, mono-cropped. This is one thread of several that I'll be putting up for advice on what can be done to try and steer this ship a little closer to the correct course (I also recognize that several type 1 errors have already been made, that can't be rectified without clearing the whole orchard and starting over - not feasible).

One of the issues is that there are several areas that the ground squirrels have decided are a great place to make a home.  The trees are planted on mounds about 1.5' high, and it looks like they are digging tunnels under the root system. So far none of the trees have died, but I'm feeling concerned about that outcome. I spoke with a friend who was thinking that perhaps they won't damage the trees, and in fact they may be helping with fertilizer and aeration... I'm having a hard time believing that a huge underground network of tunnels under the trees won't damage the root systems, or at the very least destabilize their foundation and cause eventual toppling.

Thoughts? and if abatement is needed, thoughts on best approach? Options considered thus far include...

-hunting: time consuming
-poison: not ideal if pursuing organic certification?
-trapping: anyone have recommendations for effective traps?
-introducing predators: definitely ideal in the long term, hard to alleviate the problem quickly with this though. Entire property is fenced. Owl boxes could be used...

Thanks!

Wes, I feel your pain.  I have voles that do the same thing in my fruit orchard and around perennials.  

I've been pouring rough 3/4" gravel into the openings, carefully, packing them back into the tunnel with a stick. The rocks are jagged and they pack well together.  My local place has granite.   If the squirrels can move 3/4" gravel, then maybe 1-1/4" gravel would be better.   I made a wooden bin at the entrance to the garden where I shovel 3/4 of a ton into it (the size my pickup can haul), and I'm on my third pile.  Bucket by bucket it gets done, it's not too bad.  I wouldn't have fruit trees, blackberries or grapevines if I hadn't done this.

If they are still working under the gravel, it sinks, and you'll see where you need to add more.   Sometimes you'll see a new air hole through the rocks, knowing there was no hole there before.  Just touching those rocks ought to make them fall, and fill in with more.

I've done this for at least a year, and I'm happy with the results.

Have you checked with a critter camera to make sure it's squirrels?   I haven't had any choice but to kill the rodents.  Voles have litters every 23 days, and I can't keep up with them if I don't.   The real helpers are foxes, who have small territories and will stay in your zone.  Although they do love fruit, climb the trees and eat it.  I keep them out of the garden with an 8' chicken wire fence that they try and try to get under, but so far I'm ahead of them.

The worst issue with any rodent that digs  a tunnel is that there is now air around the roots.  Even if they don't eat the roots, it's almost impossible to get enough water to stay around those roots.  Andother rodents will use those tunnels that do eat roots.  The plants seem to do just fine with that kind of gravel under their roots, and to some degree the granite has a minerals they can use as well.

Also....not sure where you are on the coast, but if you have clay, and it's drying out this time of year and opening up the gopher/vole runs in the ground to the point where you can see where all the runs are going, the gravel can go into those open runs.  I've filled in around my fruit trees 2 feet in all directions where there are runs, and around grapevines and blackberries about 18".  Some runs get used more than others, and I'll fill those in farther out.  

I've been using this same sized rock to cover my clay soil around the bases of the plants to keep the sun off of it, to not give the rodents a place to hide under mulch, and to drop into any places they may be digging so I get a warning before it's too late.

And as you probably know, avocadoes always do better with lots and lots of leaf mulch, particularly their own leaves.  But not so close to the trunk that the rodents can hang out there and chew, and girdle the bottom of the trunk.

Thanks Cristo for the input!

The rock sounds like an effective but labor intensive and somewhat expensive solution. And of course, the only true long-term ecological solution would be to have a complete food chain present, which is unattainable due to the fact that the forest has already been removed, the cleared section fenced off, and a monocrop of fruit trees planted. So I know that there is going to be some level of maintenance and energy input for the life of this orchard required... I guess the idea of adding a lot of rock into the soil of the orchard feels a little off-putting.

Do you know the original rationale for building on raised mounds? Can you fill in between the mounds so the trees are no longer raised, even if gradually over the course of one or two growing seasons?

Wes, where I am 3/4 of a ton of 3/4" granite gravel is $30.    It is surprisingly not a lot of work.   I walk past my plants often anyway.  If there are signs of rodent damage, the tunnels have to be filled in somehow, they are way too big to leave there.  They will just dig out the dirt again, or another rodent will.   The gravel is a permanent solution.  Carrying a bucket along, or positioning several in different places in the garden make it easy when you see something going on.  

I've tried traps in the past, and there aren't enough hours in the day to clean and set traps, it's a lousy job.   I've struggled with rodents for 30 years in my rural location, and this has worked the best so far.

You can try plant barriers, like garlic, asparagus, day lillies and daffodil bulbs, but those are expensive and need extra water.  Rocks are just there forever!

These are as close to Pet Rocks as I've ever come, sure glad they are there.

There are videos on Youtube of a setup called Burrow Blocker that fills the holes with a water/sand slurry.

https://youtu.be/trwxVWtH79U

You could also do this on a small scale by the 5 gallon bucketful.
Wonder if ground squirrels make good avocado fertilizer?