From Thomas Watson’s Sermon, The Spiritual Watch:
Keep your heart as you would keep a garden. Your heart is a garden (Song of Solomon 4:12); weed all sin out of your heart. Among the flowers of the heart, weeds will be growing—the weeds of pride, malice, and covetousness: these grow without planting and cultivating. Therefore be weeding your heart daily by prayer, examination, and repentance.
Weeds hinder the herbs and flowers from growing; the weeds of corruption—hinder the growth of grace. Where the weed of unbelief grows—it hinders the flower of faith from growing.
Weeds spoil the walkways. Christ will not walk in a heart overgrown with weeds and briars. Christ was sometimes among the lilies (Song of Solomon 6:3)—but never among the thistles.