its a personal choice re the verbena. I tend to leave mine until early spring as some birds will feed on the seed. They will however self seed so you will find many seedlings come mid spring.

as for the leaves, it makes no difference as the leaves break down in the light in nature.

I do the opposite to Eileen. I cut down my Verbena bonariensis to make it bushier in the spring and stop them from germinating all over the garden. I also pack my fallen leaves in old compost bags, water them, tie them up and store them behind my summerhouse for about 18 months.

I also find my verbena grow bushier when they restart into growth in spring Jimmy. I often lift seedlings, pot them on and then sell them for charity.

My leaves, which are predominately beech take up to 3 yrs to break down. Some are in bags others in bins and others in chicken wire mesh containers. All take about the same time.

I meant that clear or dark bags make no difference to how the leaves break down.