Why Are My Asparagus Ferns Dying Back in Summer?

Asparagus ferns should stay green and healthy from their emergence after harvest through to autumn — ideally until October or even November in mild conditions. Ferns dying back in July or August, or yellowing and browning in the height of summer, are a problem because the entire purpose of the fern is to feed the crown for next spring. Every week of healthy summer fern growth matters for next year's harvest. Premature die-back robs the crown of weeks of energy storage.

Asparagus rust is the most common cause

Asparagus rust (Puccinia asparagi) typically appears in midsummer as orange, yellow or brown pustules on the stems and fine foliage. In a bad year, rust can cause the ferns to yellow and die back by August, cutting the crown-charging season short by six weeks or more. Rust is worst in warm, humid weather with poor air circulation between ferns. Apply copper-based fungicide at first sign of rust and improve air flow by not letting ferns from adjacent crowns tangle together. Clear infected fern material in autumn and bin it rather than composting.

Asparagus beetle defoliation

Asparagus beetle larvae feed voraciously on fern foliage and can strip individual stems completely bare by midsummer. A heavily defoliated fern turns brown and dies back, which looks similar to disease but has a distinct cause. Inspect the stems closely — if you can see the striped adult beetles or the grey-black larvae, this is beetle damage. Hand-pick larvae daily during the peak feeding period in June and July. Severe infestations may warrant an appropriate insecticide applied in the evening to protect pollinators.

Waterlogging and root stress

Ferns that yellow and die from the base upward, particularly in a wet summer or after a period of heavy rain, may be suffering from waterlogged roots. When the crown roots cannot access oxygen, the ferns decline even though there is plenty of water at soil level. If the bed is in naturally wet ground, this may happen every wet season and is a site problem that will need addressing — either through improved drainage or moving the bed to a better-drained position long term.

Severe drought

At the other extreme, prolonged drought in summer can cause ferns to yellow and die back early as the plant shuts down to conserve water. A dry summer without irrigation will shorten the effective fern season. Mulching the bed well in spring retains moisture significantly. In a drought, a thorough weekly watering of the fern bed is worthwhile to protect the crown-charging process through the driest months.