IIRC, the minimum is 12", but in some areas where the ground is prone to frost heaves, they can go as far as 48". You should have no problem with going 6" with flowers.
With HAND tools, not including a crowbar or spade... Don't go over the area with a freaking gas-powered rototiller to dig up the bed. And don't plant any tree, or any perennial with a significant root system. Especially don't plant wisteria.
You might find your phone or cable TV line only a few inches below the surface but I think they're a little more careful with power or gas. Since you know where they are just dig carefully.
I did want to plant two foundation shrubs that might reach 10 or 15' for privacy.
My power line is 10" form the fence line 100% of the short fence and the majority of the gas line is our side the flowerbed line but dips in where it tee's.
So with that in mind, I wonder how far on either side of the power line I could plant any shrub that loves zone 10 and partial shade. Or if it all!
If they are like most locating companies, they will REFUSE to tell you the depth. Common, good locating equipment does very well laterally but not so well with depth.
Over the years, a line is unlikely to move SIDEWAYS - short of a 6.5 Richter event.
Depth, however, can and does change. In my service area, we have a cable that is 11-feet deep. We didn't place it that deep. Rather, we buried it
30-inches deep and, later, FILL was added atop.
Conversely, something that was placed 3-feet deep is only 6-INCHES deep after they scrape off most of the cover.
Generally, you are required to carefully HAND DIG within 18-inches of either side of the locate mark. Either dig as deep as you'll be going OR expose the buried line before proceeding mechanically.
The minimum depth is generally based on the local frostline and will vary from one area to another. You can ask your localutilit companies but I can't imagine that 12" for a flower bed would be a problem anywhere for gas or power lines. Now, if you have buried television cable or phone lines, all bets are off.
had a fella do 'dirt work' on my folks' yard back in 2001. He hit the cable line about 10 places, but didn't hit gas or electric... I'd imagine that guy knew exactly how deep they were and didn't worry in the slightest.
He ran a tiller box too... and those dig fairly deep.
CALL YOUR ELECTRIC COMPANY. Why do you want to DIY something that could make you RIP? Everyone needs a little help sometimes. This is one of those cases.
Im also considering utilizing LARGE pots and containers for a lot of the area over the power line. It kinda sucks that the best part of my yard is littered with gas, cable, power and phone!
Im going to cook up a little drawing for some help planning.
I figure though if I follow this idea. 6" deep in the ground, 2 to 3 foot retaining wall, large containers for large shrubs and smaller plants in the soil for cover and maybe even ground cover like asian jasamine, mixed with a small pond built above ground, it could turn out real nice.
On the drawing, to the right side, you will see my bed layout and line layout. The only thing right now that actually exists is the porch and the bed to the left of the porch and the shed.
The green circles in the corner are where i would like to have tall shrubs for privacy. If I have to use large containers, I will.
In my area code requires a minimum of 18 inches. That may not be the same in your area. My phone and cable, on the other hand, were basically laid on the ground and had a little soil raked over them.
I think (but not sure) my power cables are quite a bit lower than this (the extreme cold tends to shift the ground visibly). For the cable and phone lines they use a tool that slits the sod and stuffs the cable underneath. Then they just walk back over it to compress everything back together.
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