It is often tricky to find weed killer that won't destroy the plant life you'd prefer to keep. Herbicides are often deadly to many more varieties than those you'd like to kill, and they can't distinguish between weeds and other plants. Thus, it is vital to select an herbicide that won't harm your trees.
Choosing an Herbicide
Ortho Long Season Weed Killer is a safe choice for most applications. It won't harm trees -- even young saplings. However, this particular herbicide won't be strong enough to kill extremely persistent weeds such as poison oak or poison ivy.
If you have weeds that don't respond well to Ortho, try Round Up. Do not apply Round Up too close to trees, as it may cause some damage to trunks and roots. Instead, apply it directly to weeds. Drip or paint the herbicide onto weeds. Spraying may spread the liquid and cause damage.
Casoron is a pre-emergent herbicide, which controls weeds before they sprout. Apply it in the fall, but do not use near evergreens or near saplings less than 1 year old.
Preparing to use Herbicide
Identify dominant weeds, if possible. Choose an herbicide that is proven to work on the weeds you need to kill. Mulch the base of young saplings to prevent weed growth close to the trunks. Shield the trunk of young trees with a barrier such as cling film, or use a sprayer with a shield.
When using spray, protect the roots and trunk from herbicide which may drift through the air. Use a paint brush or dropper to apply herbicide to weeds growing very close to trees. Apply granular herbicides carefully, ensuring the granules don't get too close to roots and trunks.
Apply pre-emergent herbicides in the fall to prevent new weeds from forming. Watch carefully for new growth and use spot treatment to destroy emerging weeds while they are still young and weak.