[prakash_mib]

I just thought that I might grow some asparagus this year (and for future) I dont want them to spread all over the plot. so planning to dig sort of 4 square feet hole, wall it with plastic/weed membrane and refill it back to make it a bed. now the question is can/should I do that?
if yes how deep should the wall be?

 

[JayG]

Individual asparagus plants don't spread themselves in the same way that plants like raspberries do, although they could potentially seed themselves if you have male and female plants (not a big problem though in practice I've found.)

There's therefore no need to try and contain them, just be aware that after 5 years or more you should have an underground root system at least 5' across! 

[Goosegirl]

Roots 5' across? Crikey!!! My 10 plants are in a 2' high raised bed roughly 4' by 20'. Depending on your soil that is the best option if it is heavy, and I would add grit for drainage. Is your hole 4' x 4' - look on here for your planting distances (just forgotten now) - depends on how many you want - you may need a bigger hole especially as has been said that they don't spread. I resisted harvesting a spear off each plant last year (2nd year in the ground) but this year ...mmmmm!!

[gobs]

I just thought that I might grow some asparagus this year (and for future) I dont want them to spread all over the plot. so planning to dig sort of 4 square feet hole, wall it with plastic/weed membrane and refill it back to make it a bed. now the question is can/should I do that?
if yes how deep should the wall be?

 

You shouldn't do that. They are not gonna spread in any way or fashion.

Just plant them in deep deep enough trench for roots. End of.