The 3rd-level Sor/Wiz spell shrink item [trans] (PH 279) works on normal plants. That is, according to the spell, the caster can
shrink one nonmagical item (if it is within the size limit) to 1/16 of its normal size in each dimension (to about 1/4,000 the original volume and mass). This change effectively reduces the object’s size by four categories (for instance, from Large to Diminutive). Optionally, you can also change its now shrunken composition to a clothlike one. Objects changed by a shrink item spell can be returned to normal composition and size merely by tossing them onto any solid surface or by a word of command from the original caster.
Further, according to the Monster Manual Glossary on the type plant, an object can include an everyday lawn:
Note that regular plants, such as one finds growing in gardens and fields, lack Wisdom and Charisma scores... and are not creatures, but objects, even though they are alive. (313)
So you just need to get some of this...
...except on a much larger scale because even 2 cu. ft. per level is a lot of lawn. Cast the spell shrink item on the lawn and when the spell entangle is needed but the DM says Nope, toss your instant lawn, speak the command word, and center the entangle on the unshrunk lawn instead.
...Or the spell secure corpse...
Although it's likely unavailable just yet, a 6th-level Clr spell secure corpse [trans] (Book of Exalted Deeds 160) causes a holy symbol to hold a corpse of any size. This would, presumably, include the remains of any destroyed plant creature. Dismissing the spell or destroying the holy symbol causes the corpse to appear near the holy symbol. Both the treant and the tendriculos can reach Gargantuan size. The DM may rule, however, that, for example, a dead shambling mound or mu spore just isn't the "[g]rasses, weeds, bushes, and... trees" necessary for the spell entangle to function. Steel yourself for sorrow.1
...Or just cast instead impeding stones
The 1st-level Drd and 1st-level Rgr spell impeding stones [trans] (Cityscape 66) is almost the spell entangle except that, among other minor differences, the spell only works on a "[b]rick, stone, or earthen surface." Use that spell when the DM says the spell entangle won't work.
Then, for the remainder of your careers, enjoy fighting creatures that fly.
1 I've thought that maybe wrapping such holy symbols in fabric under which is ball bearings and nails might then allow secure corpse corpse-filled holy symbols to be used as improvised corpse grenades, but the spell's description gives no indication that there's any force associated with the corpse materializing from the destroyed holy symbol. This both pleases and disappoints me.