Mulching—Be Careful What You Cover
Annuals can be severely injured when fresh mulch is applied too soon
The benefits of mulching are well-known. Applying mulch in late fall (mid November through December) or early spring (March) will help your plant beds immensely. However, mulching newly-planted annuals can prove fatal.
In a nutshell, please don’t mulch your annuals. Extreme injury can occur when annuals have been mulched soon after planting. Fresh mulch can be extremely toxic to young plants. It can build up toxic gases (the sour smell you might notice) and trap moisture against the stems, leading to stem rot; steal fertilizer from the soil, and even burn the plants (fresh mulch can be hot to the touch when first used).
Your best defense against weeds is to keep your flowers growing rapidly when they are young.
Do a little additional hand watering daily in the driest spots so their tiny roots don’t dry out. Remember, annuals are basically tropical weeds, and they already have a head start on other weeds in the bed. Maintain that head start by keeping them growing!