We recently had a large blue spruce tree cut down and had the stump ground. Would the resulting sawdust be a good mulch for roses?
Also, what is your opinion of using a pre-emergent herbicide for weed
control in our rose bed?
Sawdust has high carbon content and may rob soil of nitrogen and moisture. It is also recommended for acid-loving plants and may be problematic for roses. There may also be compaction problems with sawdust, so it may need to be combined with other mulching materials to improve water penetration. Sawdust also decomposes slowly and compacts (Source: Mulch It! by Stu Campbell, Storey Communications Inc., 2001).
You may also be interested in two articles by Linda Chalker-Scott. In Wood chip mulch: Landscape boon or bane, she discusses the pros and cons of wood chip mulch. She also comments on sawdust in an article called The Myth of Pretty Mulch.
If you had spruce chips, they would be fine for mulching roses. Avoid
letting mulch touch the main stem; the goal is to pile it on the root
system away from the stem. You can remove it in the spring, or at least
be sure that it’s not too deep. While mulch protects from cold in the
winter and drought in the summer, if it’s too deep, water cannot get to
the root zone of the plant.
I would recommend that you avoid chemicals, as I find that you have to pay more attention when you use them than if you just wander through the garden now and then and pull all the weeds you see.
Pre-emergent weed controls never provide complete weed control. The most important thing to do is weed the area first, as pre-emergents only control weeds that have NOT sprouted. And if you have lots of seeds in the soil, don’t expect weed killer to eliminate them all. If water is required, beware of too much water (i.e., rain) that can wash away the herbicide.
Rather than use a chemical, I would weed the area now and then apply mulch. In addition to protecting the roots and soil, the mulch will suppress weeds, possibly until spring. You will have to watch for weeds that do sprout and be sure that you don’t let them go to seed. Otherwise, you will set yourself up for lots of future weeding. Chemicals don’t really help in situations like that, as you have to time their application perfectly. Hand weeding and mulching–well timed–can work better than any herbicide.