Water_Plant_Once_A_Day.ino

#include <avr/sleep.h>
#include <avr/wdt.h>

void setup() {
  pinMode(4, OUTPUT);
}

void water(int ms) {
  digitalWrite(4, HIGH);
  delay(ms);
  digitalWrite(4, LOW);
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
}

void loop() {
  for (int i = 0; i < 10800; i++) {
    sleep_enable();
    ADCSRA &= ~(1<<ADEN);
    setup_watchdog(9);
    sleep_mode();
    ADCSRA |= (1<<ADEN);
  }
  water(3000);
}

ISR(WDT_vect) {} // Wake up.

//0=16ms, 1=32ms, 2=64ms, 3=128ms, 4=250ms, 5=500ms, 6=1sec, 7=2sec, 8=4sec, 9=8sec
void setup_watchdog(int timerPrescaler) {
  if (timerPrescaler > 9 ) timerPrescaler = 9; //Limit incoming amount to legal settings
  byte bb = timerPrescaler & 7; 
  if (timerPrescaler > 7) bb |= (1<<5); //Set the special 5th bit if necessary
  //This order of commands is important and cannot be combined
  MCUSR &= ~(1<<WDRF); //Clear the watch dog reset
  WDTCR |= (1<<WDCE) | (1<<WDE); //Set WD_change enable, set WD enable
  WDTCR = bb; //Set new watchdog timeout value
  WDTCR |= _BV(WDIE); //Set the interrupt enable, this will keep unit from resetting after each int
}

This is for ATTINY 85. Deep sleep code copied from somewhere, credit to those folks.