Growing bulbs in pots allows for lots of versatility. You can move them to different areas of the yard to maximise their beauty, store them in a cool place after they flower, and you can place them wherever you would like. However, achieving the perfect growing medium is essential for a succesful planting.
Why Garden Soil Doesn’t Work
While planting a lot of bulbs, the easiest solution is to take dozens of garden soil to fill the pots. However, it’s unwise to garden soil in pots. It compacts, which squeezes air pockets that the roots need. When watering, the soil compacts to the point of not soaking, and if it does, it drains and causes the roots to dry out too fast.
In the garden, soil disesases, weed seeeds, and pests are balanced out with beneficial organisms and natural drainage. In a container, they create real problems like uninvited guests and root diseases.
Also, soil is heavy, and excess garden soil makes it even heavier. This can create issues if you are planning to place the pots on a window, a balcony, or a place where shifting them around is supposed to be easy.
Most Ideal Potting Mix Consists Of…
Products such as these are designed specifically for planting in containers. They are not as heavy, drain freely, and still hold enough moisture that keeps plants happy in between waterings. This is important for bulbs, which do not like sitting in waterlogged situations. However, they do need adequate moisture when actively growing.
Potting mixes are made of mostly the same ingredients. Usually contains either peat or peat-free alternatives, some sand or grit, and in some cases they contain a bit of loam. They are sterile when purchased, so the user does not have to worry about weeds or diseases. Most contain some type of extra fertiliser that feeds your bulbs for a few weeks.
Potting mixes have a structure that is in important that needs to be highlighted. They stay loose and crumbly and do not clump to the point where the moisture is not allowed to drain properly. Instead of channeling down the sides of the pot, the potting mixes soak the moisture evenly.
Peat-Free Options
Most gardeners have recently have chosen peat-free compost for the environmental benefits. However, some features such as drying out slightly faster could result in needing to water the bulbs more frequently. Peat-free mixes are more prone to settlement and compaction compared to peat containing mixes.
Some peat-free composts, while on the coarser side, are made with visible wood fibre. Such coarser blends are fine for bulbs, as they are not picky. If anything, the extra aeration helps with rot prevention.
When Topsoil Makes Sense
There is one circumstance under which the addition of topsoil makes sense with container bulbs, and that is if you are mixing it with other constituents to form your bespoke mix. A mix where one-third is topsoil, one-third is potting compost, and one-third is horticultural grit will get you fairly close to a loam-based compost, e.g., John Innes.
This kind of blend has advantages. It is heavier, for one, which is a good thing in terms of stopping tall pots from blowing over in the wind. Also, the weight is a good thing in that it provides more stability for large bulbs, like the really tall lilies. Also, lightweight composts get used up rather quickly, but loam-based mixes are better at retaining nutrients if you’re keeping bulbs in the same pots for a number of years.
However, the topsoil must be of good quality, that is, properly screened topsoil and not just some random soil that is dug from the garden. And you must, included the grit, for without it, even good topsoil creates a container growing environment that is too dense.
Adding Grit
No matter what growing medium you pick, you can always add grit and improve it even further. \”Horticultural grit\” or \”Sharp sand\” (not builders sand) opens up the structure preventing waterlogging at the bottom of pots. This is especially key for tulips as they rot easily under damp conditions.
Using a volume of one fifth \”to one quarter of grit\” is a good mix. In a standard 30cm pot, that’s roughly a couple of generous handfuls. It may seem like a lot, but bulbs really appreciate the extra drainage.
Special Considerations for Different Bulbs
Lilies benefit from a richer mix with more nutriens. If you’re keeping them in pots for multiple seasons, use a loam based compost or add some topsoil to the mix.
Dahlia summer bulbs grow considerably quicker than spring bulbs. Spring bulbs only grow for six to eight weeks. Dahlias will grow rapidly and flower for several months. Therefore, dahlias need more nutrients than spring bulbs. Summer bulbs need more feeding and prefer a compost with more body to it.
Nerines, and smaller allium Mediterranean bulbs, prefer dryer compost. For these, increase the grit ratio to one-third of the mix.
Refreshing Compost
If you are reusing pots for the previous season, remove the top half of the compost and replace it with fresh compost. Old compost will lose structure and nutrients, but can be added to garden borders for additional nutrients.
For permanent bulb plantings that are staying in the same pot, remove the top few centimeters of compost and replace it with new, homogenous compost that is mixed with a slow-release fertiliser.
The Bottom Line
For most of the bulbs in containers, good quality multipurpose compost (preferably peat free) with extra grit mixed in works well. It isn’t rocket science, and bulbs are typically quite forgiving. Get the drainage right, and you’re most of the way there.