Ya’akov had a lot of other things to take care of when he came down to Mitzrayim. He had to oversee Yehudah’s setting up a yeshiva. He had at least one audience with Pharoah. He reunited with Yosef. Undoubtedly, then and in the years that followed he also did the things he always had done – he must have been learning, giving shiurim, inspiring his children and grandchildren in their avodah. Why did he take the time to go out in the fields and start planting trees? “Lechteiach acharai bamidbar b’eretz lo zeru’ah…” Bnei Yisrael are praised for their willingness to leave Mitzrayim and go out to a desert with no food, no preparation, nothing other than their faith in G-d. Have you ever seen a Jewish family out on a trip without a shopping bag of food and nosh? This was probably the one time in history that it happened. G-d provided mon to eat, a be’er to drink from, etc. If they needed wood, it's fair to assume he would have provided wood as well. Yes, you could be mechaleik and say that because there was a mitzvah to build a Mishkan they had to prepare mi'din hechsher mitzvah, but still the question remains, why was Ya’akov so concerned about it years in advance?
R’ Avraham Yafen answers that Ya’akov wasn’t just planting trees – he was planting emunah.
When you are in the midst of a long, harsh galus it can seem like there will never be and end. Ya’akov could have spoken to his children and grandchildren about the promise of geulah, but let’s be real: even if your Rabbi gave a speech every week that the geulah was coming, who would listen? Who would take it seriously? Words, words, and more words. But imagine if you met your Rabbi at the travel agent booking a one way trip, open ticket, to Eretz Yisrael. You walk into his house and his suitcase is packed, his furniture sold.
“Rabbi, did you decide to quit? Are you leaving us?
“No, I’m still your Rabbi, but I’m preparing for geulah and want to book my flight before all the tickets are snatched up.”
Suddenly it’s real, it's a message you need to take seriously. Lots of people talked about moshiach coming before the Chofetz Chaim, but the Chofetz Chaim lived with his suitcase packed -- he made it real. Ya’akov personally went out to plant the trees well in advance of the need for them because he wanted the vision of geulah and the Mishkan to not just be a derasha, a promise, a vision of the future -- he wanted it to be part of the reality that Klal Yisrael would live with.