A 75-year-old Perth pensioner who grew cannabis plants to treat her mentally ill son has received a suspended jail term of 10 months.

Miriam Down was charged last year after she was found with 19 cannabis plants in her Bayswater garden, as well as 1.4 kilograms of cannabis leaf in her home.

Ms Down, a nurse, told police she was using the cannabis to treat her adult son George, who has bipolar disorder and affective schizoid disorder.

She said the drug had significantly improved George's previously violent behaviour and allowed her to feel safe.

Ms Down was handed the suspended prison term during a sentencing hearing in the District Court after being charged with possessing and cultivating cannabis with the intent to supply.

Her 53-year-old tenant, Judith Robinson, was also charged for assisting Ms Down to grow the plants and was given a 10-month intensive supervision order for her involvement.

District Court Judge Vicki Stewart said there was no commercial gain involved for Ms Down, and described her crime as "a misguided attempt" to assist her son.

Carer duties had 'ruinous effect' on Down

The court heard Ms Down's son had been receiving psychiatric treatment for more than 20 years, and she began supplying him with cannabis to calm him down.

The judge accepted Ms Down had cultivated the cannabis plants purely for medicinal use. ( ABC News )

Ms Robinson, who rents Ms Down's granny flat with her partner and has a background in horticulture, saw the plants were not thriving and provided advice on how to better water and fertilise them.

Lawyer Shash Nigam said Ms Down believed her son suffered whenever he was admitted to psychiatric facilities, including Graylands Hospital, so she cared for him at home.

He told the court Ms Down's caring responsibilities had a "ruinous effect" on her life, preventing her from attending church or engaging with friends, and even causing her to miss her brother's funeral.

"All her energy and all her time is spent managing her son," Mr Nigam said.

Judge Stewart said the number of cannabis plants was "not insignificant", but accepted Ms Down had cultivated them purely for medicinal use.

However, she said the pensioner should not have attempted to help her son by supplying him with cannabis.

"His psychiatrist, not you or anyone else, is in charge of his mental health," she said.

Judge Stewart acknowledged Ms Down's financial situation was "not good" and she would not be able to pay a fine.

The pensioner received a 25 per cent reduction on her sentence for her guilty plea and cooperation with police.

Cannabis makes way for flowers and vegies

After her sentence was read out, Ms Down told the court she would never grow cannabis again.

"I've got flowers and vegetables out there now," she said.

Speaking outside court, Ms Down said she was "happy".

"I think the judge was very fair. I hate drug traffickers," she said.

Ms Down, a widow with three sons, retired when she was 48 and became a carer for her mother-in-law, mother and husband.

She said her son, who is now in rehabilitation, had been violent before the cannabis use, smashing up her house and throwing food around.

"After a while, you get desperate," she said.

Ms Down still faces the prospect of losing her house under the Criminal Property Confiscation Act.

Her lawyer, Mr Nigam, said he expected to hear from the Director of Public Prosecutions on the matter next year.