You need the dirty beaker from inside the museum om Main Street (upper right).
Notice that there are 9 lines on the beaker. In the Research Lab, fill the Dirty Beaker up with 3 lines blue, 2 lines green, and 4 lines red.
The Weed Whacker is an attachment to the Mech that Zack is building in the garage next to Sully's place. When you have recovered the two parts needed to finish the Mech (the Steam Motor and the Crank), you can complete its construction and add the Weed Whacker onto it. This will let you clear the mass of plants from the annex (storeroom) entrance at the far left of the island.
Once the Mech is complete, click on the Weed Whacker to install it. When you reach the overgrown entrance, hit your "spacebar" to make the Whacker work. You can aim it up and down with your mouse, and move the Mech back and forth to reach all of the vines. Three or four properly aimed thrusts should clear the doorway.
Inside the Annex, you will use the Weed Whacker to "splat" attacking plant creatures, and to knock open a stuck Giant Fan at the final entrance.
(see related question)
Use the Weed Whacker on the weeds by pressing Spacebar.
After building the mech (get the motor and crank), climb aboard the Mech and select Equip on the Weed Whacker on your items. To use the Weed Whacker to "splat" plants, aim up or down with your mouse and press the spacebar.
Once you have attached the Mech Crank and the motor, enable the Mech Weed Whacker and use the spacebar to whack the weeds (you need at least two hits per section on weed growth).
Clearing the weeds from the entrance at far left (the Storeroom) will have to wait until you finish building the Mech walker, which is in the garage next to Sully's. The weed whacker attachment for the Mech is in Captain Zigg's quarters on top of the Hub. When the Mech is finished, climb aboard and use the weed whacker card. Aim with your cursor and press the spacebar to whack weeds.
Clearing the weeds from the entrance at far left (the Storeroom) will have to wait until you finish building the Mech walker, which is in the garage next to Sully's. The weed whacker attachment for the Mech is in Captain Zigg's quarters on top of the Hub. When the Mech is finished, climb aboard and use the weed whacker card. Aim with your cursor and press the spacebar to whack weeds.
use the weed wacker. and press spacebar
you use the weed wacker
You will need the Weed Whacker from the secret closet in the Captain's Quarters of the Hub, and you will attach it to the completed Mech (next to Sully's) to use it. The mech motor is in the production zone (far right) along with the key to the Living Quarters, which you need to get the Mech Crank (far upper left at the storeroom). (see related question - this is most of the island)
use a blower or rake to get the clippings
For most of the monsters, you can use the weed-whacking attachment on your mech. If that doesn't work, shoot them with the toxic blaster attachment.
Using the code from Mayor Crumb's office (0516), you can open the secret compartment in the Captain's Quarters. You will find a Weed Whacker attachment for the Mech, but more importantly the Bridge Key. Connect the steam battery to the bridge terminal, and use the connectors to raise the pressure to 16 (a total of +6 on the dial).
u press the space bar
you use the Weed Whacker or the Toxic Blaster