Should I cut off the yellow leaves?

Yes, you can snip them off with clean scissors to help the plant focus its energy on healthy growth. It will not hurt the plant to remove them.

Does yellowing mean it needs fertilizer?

Usually no, as yellowing is almost always a water issue. Only fertilize during the active growing season if the plant is otherwise healthy.

Smallflower hairy willowherb: yellow leaves

Check these first

Feel the soil to see if it is soggy or bone dry.
Check the drainage holes to ensure they are not blocked.
Look for signs of pests on the undersides of the yellowing leaves.

Likely causes

Overwatering
Possible
How to tell: The leaves turn yellow and feel soft or mushy to the touch. You might notice the soil stays soggy for days.
What to do: Allow the top inch of soil to dry slightly before watering again. Ensure your pot has drainage holes so excess water can escape freely.
Underwatering
Possible
How to tell: The leaves turn yellow and crispy, starting from the edges inward. The soil will feel completely dry and pull away from the sides of the pot.
What to do: Water the plant thoroughly until liquid runs out of the drainage holes. Maintain a consistent schedule so the soil stays damp to the touch at all times.

Yellow leaves on other plants

Find out what's wrong with your smallflower hairy willowherb

The list above covers the usual causes. Pocket Botanist looks at a photo of your actual plant and tells you the likely cause, how serious it is, and what to try first. Identification and a health check are free.

When to worry

Worry if the yellowing spreads rapidly across the whole plant. It is normal for the oldest bottom leaves to yellow occasionally as the plant grows.

Stopping it coming back

Keep the soil consistently damp and provide good air circulation around the foliage.