Weeds take time to grow to a stage of being recognized as a threat; mustard seed & the condiment we make from it needs time to develop; leaven takes time to raise the dough. (More like sourdough than modern yeasts.) To discern what's going on involves an investment of faith, patience, compassion....not to mention all the other gifts of God.

Is it more trying for us, or for God that his Rule is thwarted by 'an enemy' sowing weeds amongst the good crop? Doesn't God seem to be able to live with it better than we can? The Preacher's natural reaction to evil in the world is to theologise in church about up-rooting it. Maybe give pastoral care. But out in our margins we have to live with it, if under protest. Discern the mustard & leaven at work as well as spot weeds. What are we to do about the problem of evil? 'Live with it' is the short answer.
Many Christians find it easier to opt for, or just slip into a dualistic approach. But putting on 'the whole armour of God' & engaging in God v Enemy spiritual warfare (& often actual!) may simply provide us with a plausible excuse to let loose the dogs of our own insecurity, aggression, etc.. And sow more weeds?

A gardening rule of thumb is that a weed is any plant growing in the wrong place. Each year I have coming up all over the place self-seedings of a flower I sowed in the right place severl years ago. Legit then, now it saps goodness from the soil & takes up space where I want to grow something else. When I look at it, I can almost see God smiling, nodding, & saying to me, "Been there, done that!"

When evil is sown by someone, somewhere, anywhere, it keeps on-sowing, self-seeding, taking on a life of its own. Those of us who sow it in the first place are the real 'enemy' in God's world garden. Don't blame some other 'Devil'! My next-door neighbour smiles at my tentative efforts to rid the garden of weeds. John zaps them all with potent weed-killer & thinks I'm mad not to do the same. He even gives me his left-over poison. Sometimes I even bring myself to use it. But it's too easy for spray to flow over onto good soil, good plants. Reminding me that if in a weak moment I try to usurp the role of God, the 'god' I become is likely to start zapping people to put an end to the evil they do. It doesn't, though, does it?

Turning to mustard seed, we may be only God's little people, but a little goes a surprisingly long way in God's hands. Someone else (the 'birds' of the story?) can build on even the littlest thing we do for God. On another tack, toast & grind mustard seed, mix it with what you fancy, & it packs a wallop! We could pack that kind of mustard wallop for God!

Some think Jesus tells the parable of the yeast as an opposite to the mustard seed, pointing out that yeast often stands for corruption [cf.16:11]. In which case, it might mean that just as good can come of the tiniest effort (mustard seed), so can corruption set in & spread just as leaven 'hidden' in flour helps expand it. Worth a thought? As a keen bread baker, I prefer to focus on the positive aspects of leaven, flour & water coming together to make a new creation. Watching the dough expand & saying, "That's good!" When it feels right, confirming the process is on track, I say "Well done, good and faithful servant..." Knocking it down & knowing it will have its Easter. (Destroy this body.....) To bake it is to smell a miracle. It's hard resisting the temptation to tear it apart & eat it on the spot. Remember to give thanks for 'this day our daily bread'.

Does anyone else ever teach as compellingly as Jesus does by his inimitable use of story? Stories enter & capture our hearts via our imagination, a very much under-rated gift of God! Preach from our head only, & we'll likely cheat our listeners as well as bore them. Heart-centred preachers, on the other hand, also do their head homework, but then express it all in heart language. There're lots of parables that haven't even begun to be told yet! Like the one about the Preacher who............!