Ivy is a welcome home for rats

Question: The yard in my new home is covered in ivy, and the ivy is just beginning to grow up a beautiful old oak tree. My neighbors claim they’ve seen a rat running into the ivy at night. What’s the best way to control the ivy and get rid of the rat?

Answer: Yikes! Your property has been invaded by at least two nonnative species. Though we haven’t seen your yard, the ivy is probably English ivy (Hedera helix) or another closely related vine such as Algerian ivy (Hedera canariensis), or Atlantic ivy (Hedera hibernica). As their names imply, these ivy plants originated in Europe or North Africa; in California, these vines are aggressive invaders.

You definitely need to keep ivy from growing on your oak tree. The weight of the growing ivy can severely weaken tree branches and the vines can provide a ladder for fire, too. A mature English ivy plant can weigh up to 2,100 pounds and have trunk-like stems that are nearly a foot thick, a cutting challenge even for a chain saw.

On top of that, a dense growth of ivy is a perfect place for brown rats (Rattus norvegicus) or roof rats (Rattus rattus) to make nests. The brown rat is larger than the roof rat and has a tail that is shorter than the combined length of its head and body. Although it’s known as the Norway rat, the brown rat is actually native to China. It came to North America on sailing ships in the late 1700s and can now be found on every continent except Antarctica. Brown rats burrow along building foundations, beneath rubbish or woodpiles and in moist areas in and around gardens and fields.

Roof rats are slightly smaller than brown rats, with tails that are longer than their heads and bodies combined. Agile climbers, they usually live and nest above ground in shrubs, trees and dense vegetation, such as ivy. Both brown and roof rats thrive year-round in our climate. They eat and contaminate food, damage structures and property and transmit parasites and diseases to other animals and humans.

So, how do you get rid of these obnoxious invaders? Let’s start with the ivy.

While it’s still young, you can pull or dig the roots of ivy plants out of the ground and around the base of trees. The best time to do this is after a rain, when the ground is moist. Remove all runners. Be sure to wear gloves when removing ivy, because some people are sensitive to its sap and can develop a rash. You also can cut the ivy with power saws or manual saws, then pry the vines off trees or structures. Ivy can re-sprout and establish itself from stem fragments, so take care to place all pieces of the removed plant off the ground, in a place where they can dry out. Dispose of them in the green waste bin, but not in your home compost pile.

While we want to avoid using herbicides whenever possible, the best time to apply herbicides to ivy plants is when the plant is actively growing and before it flowers, in late summer or early fall.

When food, water and shelter are available, rat populations can increase quickly. Therefore, the best way to manage rats is by removing food and shelter by thinning dense vegetation and creating at least a 2-foot space between each shrub and between shrubs and buildings.

Trapping is the safest and most effective method for controlling rats in and around homes, garages and other structures. Because you can use snap traps again and again, trapping is less costly than poison bait but more labor-intensive.

We especially advise against using poisonous bait to control rats that live outdoors, because pets or birds can eat the poisoned rodent and suffer a similar fate.

Here are some online resources for trapping rats and managing ivy:

How to manage pests, woody weed invaders: bit.ly/36YxviN

How to manage rats: bit.ly/3DM6FX7

English, Algerian and Atlantic ivy: bit.ly/3DQosww

Invasive plants, including ivy: bit.ly/3zt8lVa

Contributors to this week’s column were Pat Decker, Karen Felker and Robert Williams. The UC Master Gardener Program of Sonoma County sonomamg.ucanr.edu provides environmentally sustainable, science-based horticultural information to Sonoma County home gardeners. Send your gardening questions to scmgpd@gmail.com. You will receive answers to your questions either in this newspaper or from our Information Desk. You can contact the Information Desk directly at 707-565-2608 or mgsonoma@ucanr.edu.