Thank you for your question and sorry squirrels have been bothering your garden.

Your observations on the ultrasonic repellents ineffectiveness after the novelty wore off corresponds to their ineffectiveness with mice and rats, too, unfortunately. It would be nice if they worked. 

Squirrels are a challenge!
Physical barriers are recommended over repellents, such as chicken wire “baskets” protecting pestered plants, or chicken wire laid on the soil surfaces, especially in pots, to prevent digging. Squirrels cache food, often by burying it in holes, so aim to make all the soil in places where you don’t want them to dig much, much harder to dig in than surrounding areas.  Please bear in mind that squirrels can chew through plastic netting, so advise using metal instead.

On the bright side, maybe some day a squirrel will plant an oak tree for you!

 https://extension.umd.edu/resource/squirrel-damage-trees-and-shrubs/

If you have a bird feeder, you may wish to consider removal/squirrel-proofing etc. if it is attracting squirrels to your yard. Removing/reducing food in your yard makes it less attractive to squirrels. 

There is always a balance with plants that also provide food for birds, so you might want to phase in only as much wire netting as needed to thwart the squirrels without also fully excluding the birds, too.

Interestingly, some gardeners claim success repelling squirrels from particular plants by draping the base of the plant with tulle. It would seem that squirrels would just tear through the tulle the same way they chew through plastic fruit trees covers and haven’t any evidence to support its effectiveness.

Anecdotally, I have had some good results with a water sprinkler with a motion detector that would startle garden intruders with a harmless spray of water. It is very effective with deer as long as its location is changed to keep it a surprise. Unfortunately, it was less effective with squirrels, who are a smaller target and very agile, so it was more work to keep the position novel than with deer. It might be worth a try, if you don’t mind repositioning it frequently.

Please keep us posted on your garden!

Kate McLynn, Extension team volunteer