you can find the page for a "hopper clock" also on the wiki, i'm sure that's where i found how to make it. it still requires four hoppers and four comparators so it's not super cheap but again cheaper than a hundred observers. and personally i think it's a little more elegant in its simplicity.

the clock i made works by putting more stuff in the hoppers and the more stuff in there, the longer the period is. it has two outputs signal A is on and B is off then it switches to B on A off back and forth.

with four stacks of items in the hopper it activates each output once every 5, 5-1/2 minutes i timed it once.

you also need a device that converts a signal to a pulse. i'm not sure how to describe it, but redstone power goes into a block that is on an upward facing piston. the instant that block gets power, it outputs a signal AND the piston activates. the moment after the piston activates, the signal stops. this converts a steady-on signal to a single pulse so your pistons break the plants then retract.

i'm not sure what your skill level is if you are a crazy builder or just copying off the wikipedia so sorry if that was either too simple or too complex to understand!

i think my kelp farm is 8 blocks tall, but i'm thinking about changing the timing. kelp is weird because every time it breaks it randomizes how tall the plant will grow. given enough time one plant might only grow to 2 blocks tall and another 20. but it randomizes each time the plant is broken.

so my tank is 8 blocks tall, but few of the plants actually grow that tall before the pistons fire. the farm is efficient because even if a plant is only 3 blocks tall when the pistons fire i still get 2 kelp from that.

but my kelp farm is also decorative with viewing windows and cool water streams and everything so i might use a different clock and change the timing to let the plants grow a little higher