Raised Bed Garden FAQs: 9 Common Questions

About a year ago, I wrote a post about 7 Common Mistakes in Raised Bed Gardening.  It has since become one of the most popular posts here at the Beginner’s Garden, and many readers have posted more questions about their raised bed gardens. 

So I thought I would visit this topic again and answer some of those questions. In this episode of the Beginner’s Garden Podcast and in the post below, I dive into nine of the most common questions I hear. Click below to listen to the full discussion or continue reading.

 

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1. How deep does the raised bed need to be?

The height of your raised beds will depend on your personal preference and needs. Most people choose beds between 6″ and 24″ deep. But that’s a big range, right? How do you know how deep your raised bed should be?

I talked about the height of raised beds a bit in this post about soil in the raised bed. Most of my raised beds are either 8″ or 10″ deep. They are either built with one board that is 10″ wide or with 2 boards that are each 4″ wide. If you have deeper beds, remember that you will have to fill that depth with soil and that can be difficult and expensive.

You do want them to be at least 6″ deep. This will depend on what you are planting and if your beds are directly on top of soil or something else like concrete. If your beds are on top of soil, you can more easily get away with shallower beds because the roots will grow deeper into the soil below. 

Some gardeners have 18″ tall beds or even taller due to the ease of working in the garden itself. If you choose to have taller beds like this, you’ll want to make sure that the boards are braced well and can support the weight of your soil.

2. Do I need anything at the bottom of my bed?

There are two reasons why you might ask this question. First, you want to avoid weeds from growing up and into your garden. Second, you may be worried about ground animals that might come up from under your raised beds and eat your plants. 

For weed control, most of your weeds will not grow up through your raised bed soil to the top of your garden. Weed seeds need light to sprout and, for the most part, won’t be so aggressive to grow through deep soil. Most of the weeds that you’ll have to deal with will come from the wind or birds dropping them down. They may also come from the soil itself, especially if it’s native soil and you don’t use a thick layer of mulch to block their access to light.

One common suggestion is to line the bottom of the garden with cardboard. This will not only keep out the pernicious weeds, but will benefit your garden by keeping your soil from seeping out and keeping it moist. This environment also encourages beneficial earthworms. When the cardboard starts to break down, it adds nutrients to your soil.

I would caution you against using landscape fabric. It will not break down like cardboard so it will not benefit your soil. And while it is permeable when it is new, the smaller particles in your soil can clog the holes and cause drainage problems for your bed. 

To protect your garden from pests, you can line your bed with hardware cloth or chicken wire. This would keep the moles, voles, ground squirrels and other pests from being able to burrow into your raised beds. 

3. Is it safe to plant vegetables in cinder blocks?

Modern cinder blocks no longer contain cinder; instead they contain fly ash, which is a by product of coal. The gardening and environmental world has not been able to come to a firm opinion on whether or not these blocks leach chemicals, though the biggest risk seems to come if they are chipped or broken. If you are concerned, you could use a plastic lining in your bed. Or you could save the holes of your concrete blocks for non edible plants, like marigolds or other flowers that will attract beneficial insects. 

4. Can you explain the best way to use mulch in my raised bed garden? When should I add it?

Since we more often think about mulch in our ground beds, this is an excellent question for raised beds. I wrote about mulch in this post that will be helpful for you. 

I add my wood chip mulch when my plants are about 6″ tall. You will want a layer of at least 2″ of mulch to adequately cover your soil and prevent weeds from growing. 

5. Will raised beds really help with drainage if the soil you’re putting them on doesn’t drain well?

From my personal experience, yes they do. My soil has a lot of clay so it does not drain well, but my raised beds thrive even when the ground beneath them is saturated. In fact, one of the best ways to grow a garden if you have poorly-draining soil is to build raised beds.

6. I have an old deck. Can I make raised beds out of old decking?

It depends on when the wood was manufactured. I constructed a raised bed out of an old fence, and you can see my process here. 

Most decking and fencing are made from pressure-treated wood, and here’s the deal with pressure-treated wood.  If it was manufactured prior to 2003, it was treated with CCA, Chromated Copper Arsenate. The concern is that arsenic could leach into your plants. I wouldn’t take the risk unless I covered them in food grade plastic, but that’s a personal decision. 

Personally, I don’t have a problem with using post-2003 pressure-treated wood in my raised beds. But again, that’s a personal decision.

 

7. What should I do to get rid of weeds around or outside of my raised bed garden? Is it ok to kill them by spraying a weed killer?

No, you do not want weed-killing chemicals in your soil or anywhere around our plants. The risk is great not only for your food but for the environment as we cannot control where those chemicals go.

If you have a few weeds around your bed, you could spot treat with a vinegar mixture. Just be sure that the vinegar doesn’t get anywhere on your plants.  Personally, I would take a broader approach.

There are several options for taking care of weeds around your raised beds. One option is that you could lay down cardboard or rolls of butcher paper. On top of that barrier, place some sort of mulch on top. Pine needles are an excellent choice for this, as they do not break down as quickly as other kinds of natural mulch.

I’ve also let the grass grow up to my beds and mowed as needed. It looks nice and there are many beneficial insects that live in grassy areas, like ground beetles and ground-nesting bees, that could be attracted to your garden.

8. How long can one use the dirt in the raised bed? Do you recommend replacement or a partial refreshing?

Yes, raised bed soil definitely needs to be refreshed, but a complete replacement isn’t necessary. In my own garden, I have a long growing season and I plant several different crops in each bed and they deplete nutrients from the soil. 

I recommend adding a layer of compost 2-4″ deep on top of your soil. Earthworms will help to mix it in naturally over time. It may be better to do this in the fall, though spring is fine, too. I would add the layer of compost, plant your crops right there, and then add your mulch on top. 

9. What is the best soil mixture?

I discussed this in detail in this post about soil for raised beds. While there are many options, you’ll want a mixture of topsoil, compost, and organic material. There’s not necessarily a strict recipe; it often depends on what you have available. 

Make sure you check out my free guide, Raised Bed Soil Options for Any Budget. I’ve laid out 3 levels of options depending on your budget that can help you decide how to choose the best soil option for your garden. 

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What about you? What questions and/or experiences have you had in your raised bed garden?