Question

Where can I find powerful Grass TMs in HeartGold/SoulSilver?

Answer 1

People's definitions of "strong" will vary depending on game progression and personal preference. Because it wasn't defined in the question, I have listed all five Grass TMs available in HeartGold and SoulSilver. In general, the two most conventionally useful are TM19 (Giga Drain) and TM53 (Energy Ball), but both come so late that another choice might be better depending on your situation.

TM09 (Bullet Seed)

Non-repeatable
- Route 32 (On the ledge west of Youngster Gordon)

TM19 (Giga Drain)

Non-repeatable
- Celadon City (Reward for defeating Erika)

TM22 (SolarBeam)

Repeatable
- Goldenrod Department Store 5F ($3000)

TM53 (Energy Ball)

Repeatable
- Battle Frontier (64 BP)

TM86 (Grass Knot)

Non-repeatable
- Route 11 (Northeast corner)

Repeatable
- Pickup (Levels 71-90; 1% chance)

Answer 2

There are not very many grass tms the strongest is solar beam in the

goldenrod department store but it takes up two turns. energy ball

you can find in the battle frontier store for 64 bp. giga drain you get

after defeating the grass gym in kanto.