Use "clean" plastic or ceramic pots. Terracotta pots tend to collect salts, harmful chemicals, and pests. Don't use a pot which is too large. Never move a plant to a larger pot until the roots fill the present pot. Violets tend to bloom most when they are slightly root bound.

Save those wood ashes from the fireplace and throw them in your flowerbeds at the beginning of the season.

One way of discouraging both these pests is to plant the carrots & onions in alternate rows.
Also a very old gardener once told me that carrot fly never fly more than 18" from the ground; so if you surround your carrots with an 18" high barrier it should prevent attack.

If you know you have been in contact with one of the plants, wash your skin with soap and cold water to rinse off the sap (Urushiol). Apply rubbing alcohol with cotton balls, or use alcohol wipes that are pre-packed to the parts of the skin that are affected. Make sure you wash all clothes and shoes with hot water and strong soap.