The Plant Lady: The best ways to tackle weeds, water loss and insulation

Mulching is one of the most important tasks you can do for your garden, as it encompasses water loss prevention, weed suppression, fertilization and insulation.

Preventing evapotranspiration is key during the summer. Evapotranspiration is defined as the loss of water from the soil and plants into the atmosphere. Mulch acts as a barrier against solar radiation, which has the ultimate impact of pulling moisture out of the soil. This is important in a drought-stricken climate such as ours. When mulching, the time between watering can be extended by days.

When laid on thick (3 to 5 inches), mulch can prevent weeds from growing. When they do make it through, they tend to be weaker and easier to pull out. For added weed suppression, lay down cardboard below the mulch. This is recommended for tough weeds such as Bermuda grass and bindweed.

Earthworms are a known garden helper. Their castings are rich in nutrients such as potassium, phosphorous and calcium. Additionally, they work and aerate the soil by literally eating it and excreting it out with a mixture of mucus. This creates a pliable garden soil. Earthworms require a cool environment and will often be found several inches underground. You can use mulch to entice them to work the soil closer to the surface. Mulch helps insulate soil, creating a cool environment for not only earthworms, but for garden roots as well.

Compost

Compost is the gold standard in mulch, it is decomposed green and brown organic material (nitrogen/carbon). Microorganisms, fungi, bacteria and earthworms break the components down, releasing nitrogen, phosphorous, potassium and micronutrients. Homemade compost will work — but if limited in supply, you can always purchase.

Wood chips

My go-to are wood chips from tree companies. You can often get them at a low cost, and sometimes even for free. Tree companies are good at not delivering diseased trees or black walnut, but it’s a best practice to ask beforehand. Black walnut contains juglone, a chemical that suppresses and even harms growth of plants. Juglone does break down over a few weeks, but sensitive plants can be affected by even small amounts. If you have horses, make sure that oleander is not included in the batch. If you have acidic soils, be careful of adding too many oak leaves, as they contain large amounts of tannins which can exacerbate the acidity issue.

Yes, you can use bagged wood chips -- however there are a few things to keep in mind. According to scientists at the University of Massachusetts, wood chips are made from various scrap material coming from many sources (i.e. wood pallets, building materials, etc). There has been a ban on arsenic-treated wood for years, but when sourcing scrap wood, arsenic remains a possibility. This may be something to consider, especially when used in a vegetable garden. The dyes themselves for the wood chips are iron- or carbon-based.

Straw

Make sure you are using straw as a mulch and not hay. Hay contains the seedhead of the crops harvested. This can result in having many unwanted weeds in your garden. Straw is used a lot back east during the winter to insulate plants. It can be used during the summer as well to retain soil moisture. Since it forms a pretty heavy weaved mat, it is good at suppressing weeds. Unlike wood chips and compost, it does not readily breakdown and is low in nutrients.

Personally, I find application of straw the easiest on my back. The bale is heavy — but once in place, simply pull off flakes and spread. I recommend wetting a bit directly after placing down to prevent wind from blowing it away. If pests such as earwigs are a problem, then you may want to remove the straw at the end of the growing season, as it does create a nice environment for them.

Horse Manure

I don’t recommend any other manures as a mulch, however broken down horse manure is relatively mild. Often you can find horse manure for free at ranches wanting to get rid of it. Make sure you grab from the oldest decomposed pile so it is not “hot”. Also, if not broken down completely you may end up with oats growing in your garden.

Green mulch

Green mulches are cover crops that upon harvest, instead of being placed in a compost bin — they are left on top of the soil. Generally, green mulch isn’t plentiful enough to create a thick layer, so it is a good idea to mix with a layer of wood chips or compost. Cover crops are generally those that fix nitrogen, however don’t necessarily have to be. Nitrogen fixing plants mean they have created their own nitrogen with the aid of soil bacterium (as opposed to having sourced it from the soil). When plants are cut down during bloom time the nitrogen content is at peak levels. As the cover crops break down, the nitrogen is added into the soil. Popular cover crops are legumes (including fava beans), clover and vetch. Other cover crops used for green manure that are not nitrogen-fixing are the California native flower Phacelia and members of the radish family.

In general, remember the thicker the mulch the better — no pixie dust. But when mulching around trees or woody plants, avoid placing it up against the trunk of the plant.