Which substrate can contain weed seeds?

Composted manure is a substrate that can contain weed seeds. When manure is collected and composted, it often originates from animals that may have consumed feed mixed with weed seeds, the seeds of which can survive the digestive process. If the composting process does not reach sufficiently high temperatures for a long enough duration, these seeds may remain viable and can germinate once the compost is used as a substrate in gardening or greenhouse applications.

In contrast, sawdust, pine bark, and perlite are less likely to contain viable weed seeds. Sawdust is typically from trees and does not include seeds, while pine bark is a bark product that is processed and lacks seeds as well. Perlite is a volcanic glass that is expanded through heating and also does not contain seeds. Thus, the likelihood of containing weed seeds is significantly higher in composted manure compared to these other substrates.