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Narrow-leaved milkweed (Asclepias fascicularis) is an ecologically important native plant that provides habitat and food for Monarch butterflies, bees, and other insects in California. Research on other milkweed species has shown that soil nutrients and herbivory can affect the plant’s allocation of resources (e.g., slower growth in plants attacked by herbivores), but the impact of nutrient availability and insect damage on Narrow-leaved milkweed remains unclear. In this project, we carried out a field experiment to examine whether nutrient availability and herbivore density affect the growth and flowering time of this native species. We randomly assigned plants to three groups: control, low nutrient, and high nutrient (n = 20 per group). A fertilizer mix containing three primary nutrients (nitrogen, phosphorus, and potassium) was added to both nutrient treatments. At the beginning of the experiment, we used a paint brush dipped in a biodegradable soap solution to remove aphids attached to milkweed plants. All plants were watered twice per week throughout the duration of the experiment. Aphids were removed in mid-March 2024 and new aphid infestation (presence and abundance) was recorded in late April 2024. We measured the height and diameter of above-ground vegetative growth to compare growth rate among experimental groups. We also recorded the time of appearance of flower buds (an indicator of flowering time) in all plants. Our study provides insight into the effects of nutrient supplementation and aphid abundance on plant growth and flowering time of this native species. Additionally, we discuss how milkweed plants cope with herbivore attacks in their natural environment.