Altering drainage

TheGardenerFromMars · March 2023

I'll be doing a lot of planting in May and some of the plants I want to use like good drainage (it seems like the majority of plants on sale like good drainage).

My questions are:

1. How do you make a decision about whether a site has good, moderate or bad drainage?

2. When planting new plants, what can you do to usefully improve the drainage? Can you add sand or grit to the mix of soil and compost to improve drainage, and if so, how much should you add?

The site I'll be working on is probably moderate drainage and so I'd like to shift it to good drainage if that's possible...

philippasmith2 · March 2023

Unless it's obvious from the soil surface, dig a hole where you want to plant and fill with water. If it drains away within minutes, you should be OK. If it sits for any length of time, you would need to investigate further. High water table, damaged pipes, compaction etc.
Drainage can be improved by adding grit but at what ratio will depend on the soil consistency in that particular spot and which plants you want there. Bear in mind that the soil can often vary from one part of the garden to another.

TheGardenerFromMars · March 2023

Thanks. I don't think the site has serious drainage problems, it might just be a little on the heavy side for some plants that like 'good' drainage. I suppose my question is how good is 'good' drainage

What grit do you recommend ?

Pete.8 · March 2023

It seems that most plants ideally need soil that is moist but well drained. But most plants can put up with conditions that are not ideal thankfully.
Quite where you'd find soil that remains moist but well-drained I've no idea other than a jungle floor!

Some info here will help you determine your soil type-
https://www.rhs.org.uk/soil-composts-mulches/soil-types

To improve drainage you need add organic matter.
e.g. home made compost or some sort of well rotted manure

Grit will not help with drainage in clay soil
https://www.rhs.org.uk/soil-composts-mulches/clay-soils

If you let us know which plants you're considering we may be able to offer more specific advice, once you determine you soil type.

GardenerSuze · March 2023

I am not sure it is a good idea to move away from the plants that suit your soil.
Aspect can help as a south facing border will dry out more quickly than a north.
My soil is improved clay, organic matter added over years. I accept that there is no point trying to grow silver leaved plants. Trying to break the rules as it were leads to poorly grown plants and expensive mistakes.

Slow-worm · March 2023

As Phillipa said, the hole/water thing is a good indicator. Unless it's obviously claggy or has lots of sand or stones, I'd just go ahead and plant.
I have 'moist but well drained' plants in places which get dry, or are heavy, they're fine. Plant info will be ideal conditions, but doesn't mean you can't tweak them.

bédé · March 2023

1. Do a soil analysis in various parts of the garden. Look at google, or ask me again. Anything with over 66% clay is likely to be poor draining, anything over 66% sand is likely to be fast draining.

2. Ask (very) locals.

3. Start with unfussy plants you see severla of near you (and that you like, of course). Then start to stretch the boundaries.

Fairygirl · March 2023

Grit will rarely make any difference in a bed or border unless you can afford to add tons of it. It's more useful in containers. Sand is also dodgy - the wrong stuff makes it all worse.
Organic matter is the only way to aid drainage if soil is heavy, assuming there isn't a major problem of some kind as per @philippasmith2's comment.
Most plants like adequate moisture and decent drainage. Achieving it isn't always a speedy process, but it also depends on the plants being used. Shrubs, trees and hedging will tend to cope better than annuals and perennials.

A couple of us have posted on your other thread about your soil and amending it to suit the plants @TheGardenerFromMars

Kathm · March 2023

Hope it’s ok to join this thread to ask a supplementary; If you do know your drainage is poor - would it be better to sink a plant pot filled with decent compost into the area to protect a plant? I have a large fatsia japonica in a pot and want to put it into the ground when the temperatures improve. It has survived the worst of a Scottish winter above ground. Is it more likely to be susceptible to freezing/rotting if it’s in a heavy clay area?? Could a sunken pot help??? I added 5 bags of manure to the area in October, it’s still pretty ‘sticky’.

Fairygirl · March 2023

No - it wouldn't work @Kathm. They want to be substantial plants, and pots aren't brilliant for them long term unless you keep hacking them back and keeping them small. The only way to improve clay successfully is to add organic matter, but it isn't quick. If you only added the manure in October, it's a bit too soon to see a difference.

Improved clay is a great medium for them though. What you might be better doing is creating a raised bed for it. I often do that as it's a quicker process with the conditions here. I have had them in quite heavy clay though, and they're ok, but it does help if you just give them better drainage. It doesn't have to be very high, and you can make it with whatever you're happy using - good timber, sleepers, brick/block etc.
If you don't have home made compost, you can also add some cheap, fibrous compost to the clay/manure, which helps as well

Kathm · March 2023

Thanks @Fairygirl. Will work on raised bed - and keep adding manure!

Altering drainage

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