Question
Planted several claret Ash trees along our drive, in a grassed area.   These were chosen for autumn colour.  Have been very disappointed in that these have not given the promised colour. One tree was broken in a storm and so had to be replaced, but in the meantime my husband planted the damaged tree into a large planter pot with potting mix.   This autumn non of the trees along the drive have coloured but the one in the planter pot has had wonderful colour.   Our soil is clay based, a bit of an iron pan below.  What should we feed these trees I gather in the spring time, to assist with their colouring next autumn?
Thank you for your assistance.  Bev
PS:  I live in the Manawatu, North Island. New Zealand

Answer
Sounds like it maybe a couple of things. Fertilizer being the most important.

Fertilizers, such as 10-20-10, are identified on their package by their chemical analysis.
The three numbers on the bag or container refer to the percentages of nitrogen, phosphorus, and potassium components in the fertilizer.

昑he first number in a fertilizer formula is the nitrogen content.....
Nitrogen is used by plants for producing leaf growth and greener, lusher leaves.
昑he second number in a fertilizer formula is the phosphorus content.....
Phosphorus is used by plant to increase fruit development and to produce a strong root system.
昑he third number in a fertilizer formula is the potassium (potash) content.....
Potassium is used by plants for flower color and size. It is also important to the strength of the plant.

As a rule the greener the leaf in the spring the more color in the Fall. I would recommend that you apply 10-10-10 fertilizer at the rate of  1 lb per inch of trunk diameter scattered around the tree and watered in good. Apply the fertilizer just before a rain storm and you will not need to water. Fertilize now and again in the fall. This will increase the overall health of the trees. Do not use tree spikes these tend to concentrate the fertilizer and burn the roots.

Also you mentioned the tree were in a grassy area--IF you have not done so mulch around the tree with not more than 3 inched deep of organic mulch not piled up on the trunk. This will help hold the moisture around the roots and keep the grass from getting all the fertilizer. Pine straw is a good organic mulch.