When using herbicide, yes, pre-emergent needs to be applied before weed seeds germinate. For winter annual weeds, this means application sometime in autumn or even late summer. (Product ingredients vary, so instructions regarding timing will differ and will be on the product label.)

If you wish to avoid herbicides, we have a brand-new web page about Managing Weeds Without Chemicals that provides alternatives. While most applicable to areas being converted to planting beds or for refreshing a vegetable bed, some aspects can be used for lawn weed control as well.

Boosting the lawn's vigor is the best way to reduce weed infiltration long-term, although reducing lawn overall and converting areas to planting beds or other groundcovers is a great alternative. This is especially true for areas that aren't suitable for good turf growth, where it will continue to struggle and succumb to weed invasion, such as shadier exposures, wet soils, or areas filled with tree roots.

For lawns, testing the soil is the best first step if it hasn't been done in a few years. The results will let you now if adjustments to the soil pH or fertilizer formulations used will aid the grass and support good growth and resiliency to drought and other stressors. Afterwards, annual overseeding, core-aerating, and other practices will help maintain good fescue vigor.

A collection of our web content related to lawn care can be browsed here, if helpful going forward: https://extension.umd.edu/resources/yard-garden/lawns/lawn-care-and-maintenance

If you need help identifying specific weeds so you know how and when to treat them (if using herbicide), feel free to share photo files here. You may only be able to attach three files in a reply, but you can reply more than once if more photos need to be shared. Weeds in bloom are easiest to identify, but some have enough distinct features out-of-bloom that we should be able to recognize them. In general, winter weeds will come into bloom in spring, but some can bloom earlier in mild weather.

Miri

You're welcome.