Why Are My Cover Crop Seeds Not Germinating?

A cover crop that fails to germinate is frustrating, particularly when the window for sowing is limited. The good news is that most germination failures have identifiable causes that can be fixed or avoided on the next attempt. Here are the most common reasons cover crops fail to emerge, and what to do about each one.

Soil Too Cold for the Species

The single most common cause of poor germination is sowing a species that needs warm soil into cold autumn ground. Phacelia and mustard need soil temperatures above 8–10°C to germinate reliably. Below this threshold, seeds absorb water but cannot complete germination. They may sit in the soil for weeks, then rot as fungal activity increases in cool, wet conditions.

Check the soil temperature with a thermometer if you are unsure. A quick guide: if you can comfortably hold bare soil in your hand without it feeling cold, it is warm enough for phacelia and mustard. If it feels distinctly cold, switch to winter rye or field beans, which germinate at temperatures above 1–2°C.

Poor Seed-to-Soil Contact

Seed broadcast onto a rough, lumpy, or debris-covered surface sits in air pockets and dries out rather than absorbing the soil moisture needed to trigger germination. This is particularly common after sowing into a bed that has not been cleared of old crop residues or where the surface is still loose and uncompacted after forking.

Always firm the seed in after broadcasting. Press the back of a rake across the surface, or walk across a plank laid on the bed. This small step makes a significant difference to germination percentage.

Dry Soil After Sowing

In a dry September or October, soil may look superficially moist on the surface but be dry enough at seed depth to prevent germination. Seeds absorb moisture to trigger germination — if the soil dries completely within the first day or two of sowing, germination may fail entirely. A good soak immediately after sowing in dry conditions prevents this.

Seed Buried Too Deeply

Fine-seeded species like phacelia and clover sown more than 1–2cm deep often fail to emerge. The seedling uses up its food reserves pushing through the soil and does not reach the surface with enough energy to establish. Broadcast fine seeds and rake very lightly — the seed should be barely covered, not buried.

Old or Low-Viability Seed

Cover-crop seeds stored in warm or damp conditions lose viability relatively quickly. Phacelia and mustard especially decline after two to three seasons of poor storage. If germination is patchy or absent from seed that was stored for more than a year, try a fresh batch. Reputable suppliers usually state germination percentage on the packet.

Slug and Bird Damage

Large seeds like field beans are attractive to birds, especially rooks and pigeons. If you sow field beans and none emerge, check whether they have been dug out. Netting or cloches over the bed for the first two weeks prevents this. Slugs can eat phacelia seedlings at cotyledon stage, leaving bare patches. Iron-based slug pellets applied before sowing on a bed with known slug pressure helps establishment.

What to Do If the Bed Has Failed

If it is still within the sowing window for a cold-hardy species, clear the failed crop, re-firm the surface, and resow with winter rye. If it is too late to sow anything effectively, cover the bare ground with a thick cardboard mulch weighted down with compost or wood chip — this prevents weed colonisation and maintains soil structure through winter.