Tribute in garden

I'm sorry to say I think this is an impossible feat to achieve. If you had the room, you could plant the small version of box hedging in the shape of the word mum and keep it clipped to maintain it - everything else would just become normal plant shaped, and your garden doesn't really allow for a box hedge.
What about finding a rose or similar and planting that in her memory, perhaps one that has a name that means something - many roses (and other plants) carry women's names, you might find one that has her name, whatever it is. I'd like to add, I'm sorry for your loss.
Only other thing I can think of is LEDs, (light emitting diodes) perhaps set into your wall, laid into the shape of the word, though this would, obviously, require a power supply.

Have to agree with Bamboo, I can't think of anything that could be planted in the lawn and mown over which you could use to spell out the word MUM. How about getting a container and using green sedums for the background and red ones for the letters? Even then you will have to work to keep it looking neat - a rose would be an easier option.

When my mother died we planted an acer palmatum it is a well grown being now and a good reminder.

how about a container-- we have one of my mum-in-laws and I keep it filled with seasonal plants as she used to do, a little like a friend of mine who always had flowers near her mums photo maybe ask yourself what your mum would have done or would like? hope it helps

I suppose you could plant the word out in crocus bulbs, then you'd have a tribute every spring ...???

Sorry to hear about your loss. I was thinking of crocus bulbs too. They need 6 weeks after flowering to take up energy from their leaves before you can mow the lawn again, though, so if you like a very tidy lawn it may not be suitable. And I guess the word shape might change a bit as they naturalise and increase.

My Mum enjoyed Lillies which she couldnt have because of her emphysema - so I have planted up i a bed with as many varieties as I can find - maybe your Mum had a particular flower she loved that you could do the same? My Grandfather loved violas so each year I plant up some of them and just looking at them reminded me of him.

at the Christy hospital in Manchester people donate benches with a plack on them to their loved ones could you use somthing like a bird bath with her name on it or sumthing like that will be there all year round ,my condolances,

Welcome Teetonrifle sorry to hear of your loss. I agree with the others that plants wont stay as you want. I think Cliffo has the seeds of an idea for you to consider and you could light it at night.

You could mark out the words in nice stones or carefully paint it on a nice slate and inlay it in your lawn , deep enough to be able to mow over.