Strong's concordance identifies three occurrences of the verb וַיִּטַּ֞ע ('to plant'), at Genesis 2:8, 9:20 and 21:33. In each case, it seems the reference was to living plants and not just to seeds alone. After planting the garden, God puts Adam in the garden, by which the reader will assume that there are living plants there.
Genesis 2:9 repeats the account of God planting the garden, this time saying that God made the trees to grow in the garden. The two statements are interrupted by the explanation that God placed Adam in the garden, thus allowing for a passage of time before the trees grow. However, a further repetition occurs in Genesis 2:15, where God once again places Adam in the garden. The repetition occurrences are poetic and there is no suggestion of time passing between one and the next, so that 2:8a and 2:9 both describe the event of God planting the garden and making it grow, and 2:8b and 2:15 both describe the same event of God placing Adam in the garden.
Having established that there is no passage of time between Genesis 2:8a and 2:9 (in which we know that God has made the trees grow), we can read the two together and see that the tree "sprang up" immediately. As God places Adam in the garden, God tells him that he could eat the fruit from any tree except the tree of the knowledge of good and evil. Because Adam had never before seen a tree and could only have understood God's instruction by being shown the trees in the garden, the reader knows that the trees are now full size and ready to bear fruit.