Trees of the Heart

Imaam Ibnul Qayyim (رحمه الله) said in Al Fawa'id, Pages: 239-240:

Ikhlaas and Tawheed are a tree in the heart. Its branches are the actions, and its fruits are the good life in this Dunyah and everlasting bliss in the Aakhirah.

And just as the fruits of Jannah are neither cut off (by change of season) nor out of reach, likewise is the case with the fruits of Tawheed and Ikhlaas.

Shirk, lies, and riyaa are a tree in the heart. Its fruits in the Dunyah are fear and anxiety, grief and distress of the breast and oppression upon the heart. Its fruits in the afterlife are the tree of zaqqoom and an everlasting punishment.

And Allaah سُبحانه وتعالى has made mention of these two trees in Surah al Ibrahim:

أَلَمْ تَرَ كَيْفَ ضَرَبَ اللّهُ مَثَلاً كَلِمَةً طَيِّبَةً كَشَجَرةٍ طَيِّبَةٍ أَصْلُهَا ثَابِتٌ وَفَرْعُهَا فِي السَّمَاء 

تُؤْتِي أُكُلَهَا كُلَّ حِينٍ بِإِذْنِ رَبِّهَا وَيَضْرِبُ اللّهُ الأَمْثَالَ لِلنَّاسِ لَعَلَّهُمْ يَتَذَكَّرُونَ

وَمَثلُ كَلِمَةٍ خَبِيثَةٍ كَشَجَرَةٍ خَبِيثَةٍ اجْتُثَّتْ مِن فَوْقِ الأَرْضِ مَا لَهَا مِن قَرَارٍ

"See you not how Allaah sets forth a parable? A goodly word as a goodly
tree, whose root is firmly fixed, and its branches (reach) to the sky
(i.e. very high). Giving its fruits at all times, by the leave of its
Lord, and Allaah sets forth parables for mankind in order that they may
remember. And the parable of an evil word is that of an evil tree
uprooted from the surface of earth, having no stability."

[Ibrahim 14:24-26]