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A total of 10 lines:

There are 3 trees per row, and to plant 10 rows, it seems that 30 trees are needed. However, there are now only 9 of them. From this, it follows that at least some trees should be planted at the intersection of several rows (mathematically called emphasis).

To do this, we can design 6 triple points (3 row intersections) and 3 4 key points (4 row intersections), as shown in the figure below.

The 10 lines they make up are: the first line, the second line, the third line, the fourth line, the fifth line, the sixth line, the seventh line, the eighth line, the ninth line, the tenth line, where , is the three key points (three line intersections), and is the four key points (four line intersections).

Obviously, there are many solutions to such a problem, and the above is only one of them.

It is such an isosceles right triangle, and a small triangle in it will do.

This is actually a math problem, you draw a five-pointed star on the paper, which is the kind of intersection made of five lines, and then you will find the five apex corners (that is, the five pinnacles) and the five intersections in the middle, and finally plant a tree on the five apex corners and five intersections, and that's it!

Form a field character, and then add a tree outside, so that the horizontal and vertical six rows of diagonal two rows are eight rows, and the one that is added is placed on the hypotenuse of two to form nine lines.

First three trees are planted in an equilateral triangle, then one at the midpoint of each side, one at the center of gravity, which is already seven, and finally one at the midpoint of the triangle formed by the line connecting each midpoint. ok.

In the figure, 9+1 points have 9 lines, and each line has 3 trees.

12 9 6

103 4 5 The drawing is not standard, let me tell you that the nine lines are 123, 345, 165, 174, 275, 376, 296, 284, 6 10 4The outer edge is an isosceles triangle, 7 is at the center point, 2, 6, 4 is the midpoint of the triangle side, and the point is that 8, 9, 10 is not on the connecting line between the top angle and the center point and the midpoint of the side opposite the top angle, so that there will be no phenomenon of 4 trees in some rows.

First draw a regular triangle, (the center point moves down 1 4) to ask for seven, and then point three on the outside, (almost into a diamond, but not into a diamond, otherwise there will be four on the diagonal) to point out nine lines.

Draw a five-pointed star on the paper, which is the kind made of five lines with crossing, and then you will find the five apex corners (that is, the five pinnacles) and the five intersections in the middle, and finally plant a tree on the five apex corners and five intersections, and that's it!

It's the kind of intersection made up of five lines, and then you will find the five top corners (that is, the five points) and the five intersection points in the middle, and finally plant trees on the five top corners and five intersection points, and that's it!

Draw a five-pointed star, plant one tree at each intersection, and add five corners at each corner in one, a total of ten, but this is not three trees per row.