Increasing tree numbers without reducing spacing

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

30 May 2021

A reader wants to know how many trees can be planted on a 5-acre piece of land if they are spaced 6 feet apart. This is a straight forward question: all we need to know are the measurements of an acre. Unfortunately, the Kenyan acre is not equal to the standard acre.

In Kenya, a small plot measuring 50ft by 100ft is called an 8th-acre. Thus, one Kenyan acre would be made of 8 such plots. That is, it is 50 x 100 x 8 = 40,000 square feet. The real acre is bigger – 43,560sq-ft

These 8 plots may be arranged side-by-side in two rows of four each to form a large square parcel of land measuring 200ft-by-200ft. If the trees are planted on this space, each line will have 33 trees and there will be 33 lines in total. Therefore, there will be 1,089 trees in one “Kenyan acre”; which comes to 5,445 on 5 such acres.

All this is assuming that the trees will be planted all the way from boundary to boundary; but this is not how planting is done in reality. The reader did not say which type of tree he wants to plant, but, the popular eucalyptus should be planted at least 6m (20 feet) from the boundaries.

The question of how many trees will be left after allowing for this clear space depends on the perimeter (hence, the shape) of the 5-acre parcel of land. Then only way to find out is by actually measuring the length of the boundary.

If the plot is square in shape (something like Jeevanjee Garden in Nairobi), it’s sides would be 447ft long. Subtracting 20ft all the way round leaves us with 407ft for the trees. Only 68 threes will fit along this length, there will be 68 rows.

This makes a total of 4,624 trees on the 5 acres instead of the earlier 5,445. This is approximately 15 per cent loss. Interestingly, this loss can be recovered by changing the pattern of planting from a square grid to a rectangular one, while still maintaining the 6ft spacing between the trees.

I explained the details of how to do this in July 2010 (over ten years ago). The triangular pattern has about 16 per cent more trees that a square grind of the same spacing. Thus, the 5-acre square plot can fit about 5,360 trees even after leaving the 20-ft clear space at the boundary.

I must emphasise this is for a square plot. If it is not square, then the trees will be fewer than this.