This seems like a silly question, but I just can't figure it out. You've actually got a pretty tough situation there, not a silly question at all. I'm from OH and live in AL now, so my experiences are in totally different areas but the basic principles are usually fairly universal. If any of my advice is impractical for your location in CA, I'm sure someone will point it out.

I'm a newbie wanting to sheet mulch my front yard lawn.

Do you intend to grow something besides grass there, or you just want mulch instead of grass so you don't have to mow anymore? Advice I might offer would be the same either way, but I'm curious.

My little plots are somewhat small Plots - of grass, right?

Its hard to tell in the pic, but the grass slopes somewhat steeply down to the sidewalk,

I think that's from the base and roots of the tree maturing and rising.

...meaning anything I put there will most surely end up on the concrete. Do I put in edging and hope no one kicks it down so I can pile things inside? Should I dig the whole area out? Dig a trench along the borders? Any advice is appreciated!

It's not usually possible, and almost certainly harmful to a tree to start digging around its' base on a large scale. The accepted general wisdom is that it's a very bad idea to pile (more than around 4 inches) a bunch of anything at the base of a tree (pretty much your whole yard.) If you were to trench around the borders, you would probably sever almost every large root holding this tree in place, making it unstable, dangerous, and eventually dead before its' time is due.

It's hard to imagine a remedy that doesn't involve some type of containment border so I would consider bricks or some type of concrete pavers that can just sit on the surface and give you a "lip" of a few inches. You can smother the grass with cardboard which overlaps at the edges by at least 6 inches. Add about an inch of compost (if you want to add plants, not really necessary if you don't,) and about 3 inches of mulch. If you keep the mulch layer renewed when it dissipates from decomposition, your soil will become dark, rich, and well-drained over time. It's also necessary to maintain its' ability to suppress/prevent weeds.

If you want to add plants, get small ones so you don't have to attempt to dig big holes through the tree roots. When I've started beds under large trees, the roots often determine plant placement if I encounter a big one. I find it easier to scratch with one of those bent fork things than to try to dig.

It would also be good to investigate if your tree has any allelopathic (poisonous) properties that might prevent other plants from growing within its' dripline before you spend any money on plants.

Is the tree dead? It's shadow looks pretty stark. If it is dead, or you want it dead, why not cut it down? Offer it for firewood? "Bring your own chainsaw"?

You could peel off the grass around the tree trunk to kind of level the ground, then lay the peeled sod at the edge of the lawn. I would start with cardboard, personally. It shuts out the light and helps to kill the grass. Then pile stuff on top of all of it.

If passersby kick the mulch away, edge the lawn with some kind of wire mesh fencing to help contain it.

Sue

I was planning on planting edible shrubs and plants instead of grass and also wanted to contour in some tiny swales to help hold water, but I didn't realize the tree roots would be such a big problem! There are only two places that I can see the roots at the surface of the soil and I was planning to work around them, but if more are just under the surface, then I'll have a problem!

I'm currently stockpiling big sheets of cardboard that I find at work. Would it be okay to lay a thin sheet of cardboard and mulch around the tree?

The tree is a bigleaf maple and it looks sparse cause the previous owners of my house had it pruned less than a year ago. Its growing quite lush now and I couldn't get my arms around it if I tried. I would love to have more sunlight in the yard, but wouldn't feel right killing such an old tree.

Mulching right up to the tree trunk isn't usually a good idea, it promotes disease and rodent/insect damage.

Bigleaf maples have shallow, wide-spreading root systems and grow to 50-100 ft tall, so eventually it will take over that tiny yard, and will probably suck the moisture and nutrients from anything you plant in the entire front yard, not to mention shading out all the sun.

It may not do your paving and house foundation any good, either, as close as it is.

Okay, so how's your backyard???

Sue

My backyard is about the size of a large walk in closet and faces north! :(

Do you think I should have an aborist look the maple for removal? I live in a historic neighborhood and removing old trees is really frowned upon; but because its on my property, I think I could do it without a permit. Would it be costly?
Here's a better pic of the tree about 4 months ago. I think the previous owners had it topped:
{{gwi:1141674}}

My food forest dreams are dying! :(

The problem is that the tree was planted in a space that is much too small for it. They sure made a mess of it, didn't they?

I have the same problem: the original idiots here planted 12" Douglas Fir trees in a row just inside my property line, but only three feet apart. These are trees that easily get more than 250' tall.

I just don't understand why people do that. If they wanted a tree that didn't get over 12 ft tall, they should have done a little homework and some judicious selection.

Sue

from what i see there may not be a lot that can be done, if the tree can be removed i'd suggest a raised bed, another thing i see is do you have termites in your area? if so part of termite protection is not to have gardens/plants up against the house foundations, there should be a plant free zone of around 2 meters out.

len

Here is a link that might be useful: lens garden page

If there was an award for "most badly pruned tree" that one would win. That's sad.

Totally agree with belgian pup about not putting huge trees in small yards. This tree looks like it's much older than the house, though. It's provided a lot of shade (until it was mutilated) over the years but it may need to go. Calling an arborist might be a good idea.

After the tree is removed you can design your garden space.

I'd suggest building symmetrical wooden raised beds with trellis' so that you can have a stylish front yard edible garden that won't cause complaints. Make sure you find out if there are any city codes about gardens.

Give yourself at least 3' walkways between the beds because maturing plants will lean out to reduce size of the path including the sidewalk, so make sure you're not placing your gardens right up to it.

If you have a friend who is experienced with building things they could take a look at your site, use a level & help you figure it out. Wooden sides aren't absolute, but they help contain the soil & dress it up a bit for front yards.

Here is a link that might be useful: The Edible Front Yard: The Mow-Less, Grow-More Plan for a Beautiful, Bountiful Garden

That's one seriously brutalised, inappropriately planted tree.
What direction are we looking at in the photo and what sort of dimensions are the areas?
If the house casts much shade, you'll have to be realistic about what will grow, tree roots or not.
I'm really not a fan of raised beds for many reasons, but well-constructed 'edges' keep gardens attractive and stop mulch etc from 'migrating' onto the footpath.