|pic from https://commons.wikimedia.org/wiki/File:Centenarian_olive_tree_1_(4752183682).jpg|
Rav Goldwicht stated this in a special Tu B'Shvat shiur he dleivered this Sunday in NYC on the occasion of the yahrzeit of Rabbi Copperman (see http://kallahmagazine.blogspot.jp/2016/01/the-life-of-rabbi-copperman.html). He connected that halacha to the blessing for a household expressed in Tehillim 128:3
|Your wife will be as a fruitful vine in the innermost parts of your house; your children will be like olive shoots around your table.||גאֶשְׁתְּךָ | כְּגֶפֶן פֹּרִיָּה בְּיַרְכְּתֵי בֵיתֶךָ בָּנֶיךָ כִּשְׁתִלֵי זֵיתִים סָבִיב לְשֻׁלְחָנֶךָ:|
When they were taking leave of one another, Rav Naḥman said to Rabbi Yitzḥak: Master, give me a blessing. Rabbi Yitzḥak said to him: I will tell you a parable. To what is this matter comparable? It is comparable to one who was walking through a desert and who was hungry, tired, and thirsty. And he found a tree whose fruits were sweet and whose shade was pleasant, and a stream of water flowed beneath it. He ate from the fruits of the tree, drank from the water in the stream, and sat in the shade of the tree.
And when he wished to leave, he said: Tree, tree, with what shall I bless you? If I say to you that your fruits should be sweet, your fruits are already sweet; if I say that your shade should be pleasant, your shade is already pleasant; if I say that a stream of water should flow beneath you, a stream of water already flows beneath you. Rather, I will bless you as follows: May it be G-d’s will that all saplings which they plant from you become like you. So it is with you. With what shall I bless you? If I bless you with Torah, you already have Torah; if I bless you with wealth, you already have wealth; if I bless you with children, you already have children. Rather, may it be G-d’s will that your offspring be like you.